Instead, you can use an instance of a NetworkConnector with
`Request::with_connector`. This allows overloading of the NetworkStream
constructors, so that it is easy to modify how an `HttpStream` is
created, while still relying on the rest of the stream implementation.
BREAKING CHANGE
Also adds an EmptyWriter, used for GET and HEAD requests,
which will return an io::ShortWrite error if the user ever tries
to write to a GET or HEAD request.
Closes#77
