Support credentials

tl;dr
- This commit add support for fetch.
- pretender swap native fetch related API if exists
- pretender.shutdown() restore native fetch related API
- doesn't work with AbortController

Pretender has been working well with xhr, but doesn't handle fetch.
It's been 2 years since @rwjblue first open the
[issue](pretenderjs#60) for fetch
support. So people don't need to do extra work to polyfill for testing.

Include a fetch ponyfill and swap the native fetch during pretender
creation, then restore them when `shutdown`. Since fetch polyfill uses
xhr behind the scene, pretender will "just work".

Per Yetch homepage, the supplement set of yetch impl and spec include:
- Inability to [set the redirect mode](github/fetch#137)
- Inability to [change the cache directive](github/fetch#438 (comment))
- Inability to [disable same-origin cookies](github/fetch#56 (comment))

- `xhr.abort()` first set state to done, finally response to a [network error](https://xhr.spec.whatwg.org/#the-abort()-method);
- [fetch](https://dom.spec.whatwg.org/#aborting-ongoing-activities) will reject promise with a new "AbortError" DOMException.

For `fake_xml_http_request` impl, the request is resolved once its state is changed to `DONE` so the `reject` is not cathed.

So the senario happens in pretender is:
1. state chagne to `DONE`, trigger resolve request
2. abort, trigger reject
3. xhr.onerror, trigger reject

The first resolve wins, error thus not rejected but an empty request is resolved.

Though polyfilled by xhr, fetch returns a Promise and is asynchronous by
nature.

tl;dr
- This commit add support for fetch.
- pretender swap native fetch related API if exists
- pretender.shutdown() restore native fetch related API
- doesn't work with AbortController

Motivation
------
Pretender has been working well with xhr, but doesn't handle fetch.
It's been 2 years since @rwjblue first open the
[issue](pretenderjs#60) for fetch
support. So people don't need to do extra work to polyfill for testing.

Changes
------
Include a fetch ponyfill and swap the native fetch during pretender
creation, then restore them when `shutdown`. Since fetch polyfill uses
xhr behind the scene, pretender should "just work".

Caveats
------
1. The supplement set of yetch impl and spec includes (not complete):
- Inability to [set the redirect mode](github/fetch#137)
- Inability to [change the cache directive](github/fetch#438 (comment))
- Inability to [disable same-origin cookies](github/fetch#56 (comment))

2. Abort
- `xhr.abort()` first set state to done, finally response to a [network error](https://xhr.spec.whatwg.org/#the-abort()-method);
- [fetch](https://dom.spec.whatwg.org/#aborting-ongoing-activities) will reject promise with a new "AbortError" DOMException.

As implemented in `fake_xml_http_request`, the request is resolved once its
state is changed to `DONE`.
So the senario happens in pretender is:
  1). state chagne to `DONE`, trigger resolve request
  2). abort, trigger reject
  3). xhr.onerror, trigger reject
The first resolve wins, error thus not rejected but an empty request is resolved.

3. Though polyfilled by xhr, fetch returns a Promise and is asynchronous by
nature.

tl;dr
- This commit adds support for fetch.
- pretender swap native fetch related API if exists
- pretender.shutdown() restore native fetch related API
- doesn't work with AbortController

Motivation
------
Pretender has been working well with xhr, but doesn't handle fetch.
It's been 2 years since @rwjblue first open the
[issue](pretenderjs#60) for fetch
support. So people don't need to do extra work to polyfill for testing.

Changes
------
Include a fetch ponyfill and swap the native fetch during pretender
creation, then restore them when `shutdown`. Since fetch polyfill uses
xhr behind the scene, pretender should "just work".

Caveats
------
1. The supplement set of yetch impl and spec includes (not complete):
- Inability to [set the redirect mode](github/fetch#137)
- Inability to [change the cache directive](github/fetch#438 (comment))
- Inability to [disable same-origin cookies](github/fetch#56 (comment))

2. Abort
- `xhr.abort()` first set state to done, finally response to a
[network error](https://xhr.spec.whatwg.org/#the-abort()-method);
- [fetch](https://dom.spec.whatwg.org/#aborting-ongoing-activities) will
reject promise with a new "AbortError" DOMException.

As implemented in `fake_xml_http_request`, the request is resolved once its
state is changed to `DONE`.
So the senario happens in pretender is:
  1). state changes to `DONE`, trigger resolve request
  2). abort, trigger reject
  3). xhr.onerror, trigger reject
The first resolve wins, error thus not rejected but an empty request is resolved.

3. Though polyfilled by xhr, fetch returns a Promise and is asynchronous by
nature.

tl;dr
- This commit adds support for fetch, fix pretenderjs#60.
- pretender swap native fetch related API if exists
- pretender.shutdown() restore native fetch related API
- doesn't work with AbortController

Changes
------
Include a fetch ponyfill and swap the native fetch during pretender
creation, then restore them when `shutdown`. Since fetch polyfill uses
xhr behind the scene, pretender should "just work".

Caveats
------
1. The supplement set of yetch impl and spec includes (not complete):
- Inability to [set the redirect mode](github/fetch#137)
- Inability to [change the cache directive](github/fetch#438 (comment))
- Inability to [disable same-origin cookies](github/fetch#56 (comment))

2. Abort
- `xhr.abort()` first set state to done, finally response to a
[network error](https://xhr.spec.whatwg.org/#the-abort()-method);
- [fetch](https://dom.spec.whatwg.org/#aborting-ongoing-activities) will
reject promise with a new "AbortError" DOMException.

As implemented in `fake_xml_http_request`, the request is resolved once its
state is changed to `DONE`.
So the scenario happens in pretender is:
  1). state changes to `DONE`, trigger resolve request
  2). abort, trigger reject
  3). xhr.onerror, trigger reject
The first resolve wins, error thus not rejected but an empty request is resolved.

3. Though polyfilled by xhr, fetch returns a Promise and is asynchronous by
nature.