No way to get status code string from server

[joshtriplett]

If an HTTP server returns a message other than the canonical one, such as to provide a more detailed error message, as far as I can tell Response provides no way to obtain that message. We should optimize for the case of no message or the canonical message, but it should be possible to get whatever message the server returned.

[jbr]

This likely requires a small change to async-h1 as well, since we currently discard that data