Add support for manipulating the headers of the proxy request via :heade...

[blakewatters]

I had the need to make some modifications to the headers of proxied requests. This patch provides support and tests for merging additional headers into the proxied request or manipulating them via Proc callback.

[ajacksified]

+1 to this request. Would very much like the ability to manipulate headers.

[nathankleyn]

+1, I actually implemented the same thing in a very basic form on my fork of this repo, but it would be fantastic to have it supported properly.