While the protocol does not allow sending data before
receiving a successful Websocket upgrade response, we
do not want to discard that data if it does come in.
We now flush messages that are specific to cowboy_http only.
Stream handlers should also flush their own specific messages
if necessary, although timeouts will be flushed regardless
of where they originate from.
Also renames the http_SUITE to old_http_SUITE to distinguish
new tests from old tests. Most old tests need to be removed
or converted eventually as they're legacy tests from Cowboy 1.0.
