Support Response.error() and Response.redirect()

Reject the Promise returned by fetch() when Request ctor throws

Fix incorrect assertion

Remove superfluous assignment

Closes #205.

Add webpack usage link.

Allow passing a Request instance to Request constructor

The fetch spec instructs that Request should throw a TypeError when
given a Request with an already consumed body, but Chrome's
implementation doesn't respect that yet.

This makes the test error more correct when the `then` block raises an
assertion error. The `catch` block needs to re-raise the same exception,
not swallow it and disguise it like another failure.

The new Request instance becomes essentially a clone of the old Request.
The original Request's body becomes unusable.

https://fetch.spec.whatwg.org/#dom-request

Properly convert undefined/null header values to strings.

Code of Conduct

Improve documentation for `fetch` caveats

Make it a bit clearer and more to-the-point. Also make it explicit that
the example workaround code is in Ruby on Rails so there is no
confusion about it possibly getting interpreted as client-side JS code.

Have response handler functions be named like verbs so it's more clear
what they do. Also rename the `json` variable to `data` to be more clear
about what it holds and to distance ourselves from the `json()` function.

In the previous example, the `form` variable was used for an actual
`<form>` DOM element. Here's it's referencing a FormData object, so use
`data` instead to make this more explicit.

The `/query` endpoint sound like a search endpoint, but those should be
fetched over GET, not POST. Instead, use a URL that sounds like some
resource should be created.

Opt into new Travis infrastructure

Implement Headers#forEach correctly