feat(fetch): Making fetch happen

[benlesh]

• Adds fromFetch implementation that uses native fetch
• Adds tests and basic documentation
• DOES NOT polyfill fetch

Overview

I think, long term, we should deprecate the ajax methods and try to move people towards fetch now that both fetch and AbortController are universally implemented (everywhere but IE) and polyfills are available.

Maintaining our own ajax implementation is challenging, and this is lot more lightweight and full featured.

[coveralls]

Pull Request Test Coverage Report for Build 8363

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.669%

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Totals
Change from base Build 8343:	0.0%
Covered Lines:	5252
Relevant Lines:	5433

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💛 - Coveralls

[benlesh]

From Core Team Meeting

• Node concerns: Node users are probably just going to use whatever they're currently using
• AbortController isn't something that can be easily polyfilled, but exists in all modern browers.
• We should add warnings to the documentation that parts of the fetch API are experimental.
• It is published under rxjs/fetch which separates it from the other functionality a bit. Also document that this is a browser-only API.
• Perhaps test it with node-fetch just to see/make sure it works?
• If fetch exists and AbortController does not, we need to add clarity about the fact that you can't really polyfill AbortController without also patching fetch to use it.
• Add examples into the documentation... include streaming example? (idea: @trxcllnt)

👍

[OliverJAsh]

🎉 I think this will close #4534

[cartant]

LGTM