Enhanced HTTP_HEADER with new helper features.
Implemented more user friendly WSF_RESPONSE
i.e allow to change the status code and the header as long as no content is really sent back to the client

This requires an addition WGI_RESPONSE, new post_commit_action: PROCEDURE [...]
Merge pull request #28 from jocelyn/flexible_response

Allow to change status code and header as long as no content is sent.
Note this includes a change in EWSGI classes related to connector and RESPONSE. This modification was required for request processing termination.