This is to avoid hitting Bitbucket rate limits for downloads.

Fixes the test suite in some older browsers that choke on it.

This makes debugging in devices without a developer console easier.

It turns out that Android 4.0 implements ArrayBuffer but no Blob.

This restores the behavior that v1.0 had, but which regressed in v1.1.0.

Android 4.0 doesn't like the `String.fromCharCode.apply` hack.

Avoids a potential race condition when the load event already fired
before a handler was attached. This was reported happening in Safari.

Fixes #353

Before, when a Request or Response was initialized with an ArrayBuffer
or TypedArray, that data was only accessible through `arrayBuffer()`
method, and if it was initialized with DataView, it wasn't accessible at
all.

Now, regardless of the original type of body, the accessors `blob()`,
`arrayBuffer()`, and `text()` always work.

This is in accordance with Section 6.4 of the spec:

    dictionary ResponseInit {
      unsigned short status = 200;
      ByteString statusText = "OK";
      HeadersInit headers;
    };

Fixes #376

Support ArrayBufferView types as POST body

Fixes #350.

This will get all the ArrayBuffer & Blob tests passing.

PhantomJS 1.9.8 seems to support ArrayBuffer & DataView, but not Blob.

DataView is one of the possible views for ArrayBuffer, which the fetch
spec says should be supported. However, IE 10-11 don't handle it well
when passed unmodified to XMLHttpRequest. As a workaround, wrap every
DataView in a Blob.

It's interesting to note that IE 10's string representation of a
DataView instance is `[object Object]` rather than `[object DataView]`.
Therefore, the only way to detect it is via `isPrototypeOf`.

Section 6.2 of the spec says that body can be BufferSource.
BufferSource is defined in https://heycam.github.io/webidl/#BufferSource:

> The BufferSource typedef is used to represent objects that are either
> themselves an ArrayBuffer or which provide a view on to an ArrayBuffer.

Furthermore, passing an ArrayBuffer directly to XMLHttpRequest is
considered deprecated and Safari throws a warning about it. This makes
it even more important to support ArrayBufferView in fetch.

The fallback for `ArrayBuffer.isView` is provided for IE 10, which
doesn't implement the method.

Spec compatibility for Request/Response constructors and cloning

Body of Request must be cloned using a value to avoid consumption of
it. Once a new body is initialized, re-initialize the old value.

Fixes #308, closes #335

Now splits by `\r\n` because that's the header line delimiter.
No tests because I can't figure out how to return a manually formatted
headers payload from Node.js HTTP server.

Fixes #422

Fixes #411

Section 6.4 of the spec:

> If init’s `headers` member is present, run these substeps:
> ... Fill r’s Headers object with init’s `headers` member.

Fixes #416

Section 6.5 of the spec:

> Let `request` be the associated request of the result of invoking
> the initial value of Request as constructor with `input` and `init`
> as arguments.

The Headers, Request, and Response suites should mainly be exercising
those classes and not invoking the `fetch` method. Most of those tests
that do have now been moved to the "fetch method" suite.

Section 6.3 of the spec:

> If `input` is a string, then run these substeps: ...
> Otherwise (`input` is a Request object), run these substeps: ...

It's worth noting that Chrome and Firefox don't implement this right
now, and will cast any non-Request object passed as 1st argument (even
null/undefined) to a string.

Improve Readme

Hopefully this will help reduce the number of invalid issues opened.

No longer recommend es6-promise due to its size and the fact it doesn't
conform to Promises/A+. Fixes #415

Fixes #393