Implement HTTP Requests test helper with Headers
Current the test helper in oonib/testhelpers/http_helpers.py does some mangling of the HTTP headers it receives leading it to not returning in the payload of the response the proper capitalization of the requests.
We should use as base the modified Headers() that was implement for Agent and use this with a twisted based HTTP server.
Such HTTP Server must get the list of HTTP headers that a client sends to it and return it in the response payload as a JSON encoded string.