Allow to set extra request headers

[matfiz]

In my use case, I need to set custom request headers while proxying the request. I've added extra_headers option that accepts a hash of headers to be added to request headers.

So the following is possible:

reverse_proxy "/test", "http://example.com/", extra_headers: {"HTTP_FOO_BAR" => "baz"}

If you find it useful, I will add description to docs, before it will be merged.

[guemidiborhane]

why isn't this already merged? Huge thanks to @matfiz