Send 'Content-Length' header at callback request

On callback request *rush* is always sending payload (errors and complete) with full info about relayed request.

It should (regarding of [rfc2616](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html)) send this header (at least)

Maybe 'Content-type':application/json should be use instead of default, but I'm not totally sure.