Add Headers iterators

[dgraham]

Adds entries, keys, and values iterator functions to Headers. Each iterator, and Headers itself, is also iterable if the browser supports Symbol.iterator. The medikoo/es6-symbol polyfill may be used in browsers without Symbol.

Background:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#iterable
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators
• https://fetch.spec.whatwg.org/#headers
• http://heycam.github.io/webidl/#idl-iterable
• http://heycam.github.io/webidl/#es-iterable

Closes #292.

/cc @lukeapage @dkfiresky

[lukeapage]

Looks good to me, thanks for adding this. Do you think entries, values, keys will be added to the spec at some point?

[dgraham]

Do you think entries, values, keys will be added to the spec at some point?

It's a little confusing, but the iterator functions are actually specified by fetch. The Headers class implements the iterable interface, which is defined in a separate specification. All specs are linked in the initial comment above.

[lukeapage]

Ah i see, thanks.