Tweak the wording of the “Read this first” section

Allow undefined Response status

This allows the Response constructor to accept a status code that's
explicitly set to undefined. Native fetch implementations in Chrome and
Firefox behave this way.

The following now works, but would previously throw an error:

    new Response('', { status: undefined })

Ensure cookies aren't sent if `credentials: omit`

If the default value of `withCredentials` is `true`, as opposed to `false`, the library will send the cookies even if `credentials: omit` is specified. This affects React Native where the default value is `true` since cross-origin concerns don't apply.

Added yarn command as option to installation

Add global replace for processing raw headers

Added safari to native fetch browser support.

Support obs-fold as header delimiter

Accept array in Headers constructor

Improve README language

Improve various aspects of the language in the README, some being typos.

https://developer.apple.com/library/prerelease/content/releasenotes/General/WhatsNewInSafari/Articles/Safari_10_1.html

Fix grammar mistake in README

Treat any non-Request arg to `new Request()` as string url

This aims to match the behavior that the IDL of the spec defines:
https://github.com/whatwg/fetch/issues/452#issuecomment-272439900

Support Tolerance Provision when parsing headers

Closes #440

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.

This makes debugging in devices without a developer console easier.

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

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

This is to avoid hitting Bitbucket rate limits for downloads.