Add support for AbortController signal passed via context

[marioestrada]

Why

Resolves #251

Description

Adding a good unit test for the abort case was tricky because of the way it's handled in line 1389:

apollo-link-rest/src/restLink.ts

Lines 1388 to 1390 in cde5f56

	.catch(err => {
	if (err.name === 'AbortError') return;
	if (err.result && err.result.errors) {

So I added a unit test using setTimeout to test that the request never fulfills, but I know it's not a great pattern to test because it adds an artificial delay to completing the tests.

Test

npm test

[apollo-cla]

@marioestrada: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[TrevinAvery]

Duplicate of #302