Support headers

[eliassjogreen]

Like #2 it will probably be an augmentation of the global Headers class like we do for JSON and it would be wise to also support the rest of the HeadersInit formats: Iterable<string[]> | Record<string, string>.

[timharek]

Fantastic! 😎 I was just looking for this 😅

[eliassjogreen]

Reopening, forgot we probably want the return headers typed too!