net/http: allow Handlers to handle http2 upgrade PRI requests
The http2 spec defines a magic string which initates an http2 session:
"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"
It was intentionally chosen to kinda look like an HTTP request, but
just different enough to break things not ready for it. This change
makes Go ready for it.
Notably: Go now accepts the request header (the prefix "PRI *
HTTP/2.0\r\n\r\n") as a valid request, even though it doesn't have a
Host header. But we now mark it as "Connection: close" and teach the
Server to never read a second request from the connection once that's
seen. If the http.Handler wants to deal with the upgrade, it has to
hijack the request, read out the "body", compare it against
"SM\r\n\r\n", and then speak http2. One of the new tests demonstrates
that hijacking.