h2: client: do not apply host header if h2 negotiated by alpn
h2: client: do not apply host header if h2 negotiated by alpn

If client negotiated over alpn, it will have done SNI.  Some
peers like youtube.com reject with 400 if you sent a host: as
well.
sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets