Mutable access to request headers

[itowlson]

Fixes #2186.

This ended up being not symmetric with Request::headers() but I couldn't see a way to get closer (e.g. iter_mut() didn't seem like it would allow for inserts). It also surfaces the implementation in what feels like a rather raw way. @rylev Please feel free to close in favour of a better way if you have one, I'd love a learning opportunity!

[rylev]

The function was added to the Response type which I don't think was intended? In any case, would it be possible to use set_header instead of having mutable access to the headers collection?

[lann]

The function was added to the Response type which I don't think was intended? In any case, would it be possible to use set_header instead of having mutable access to the headers collection?

👍 for set_header to match the behavior of RequestBuilder::header

[itowlson]

Well, doh. Thanks for the catch and guidance. Updated.