normalize headers from a proxied request so that rack doesnt choke on them

This is a preliminary patch to sanitize/normalize the output of NET::HTTP class when proxying a request.

At the moment, it only gets the first element of the array if a header value is an array. Arrays with multiple items should be joined (I can't remember what character with though..., ';'?)

There is a gist here with the failing test case:

https://gist.github.com/nhemsley/9129335