Prettified HttpClient

Not perfect yet, but way better than before. Monster commit.
HttpParser, always return deep-copies.
HttpClient, verify Content-Length in POST and PUT requests.

Content-Length must be present in POST and PUT requests and must
be <= SlotInfo::maxBodyLength() or 4MiB if no slot info was given.
HttpClient, fixed error when a client sends too much POST data
HttpClient, fixed various bugs with POST data and piping.
HttpClient, fixed pipeFromPostBody() for QProcess devices.