Few small features and code cleanup.

* Do not send sub request to obtain 401 and 403 pages, use html documents
  returned by wwwhisper (new wwwhisper feature).
* Do not rely on Rack setting Content-Length (this is not the case for
  default middleware stack for Rails app)
* Correctly iterate over response body while injecting iframe.
* Do not inject iframe for few tricky respone types (chunked, md5ed,
  compressed)
* More private methods.