http3: discard body from responses to HEAD requests (#4115)

* http3: HEAD method should not have a body

* add tests

* Update http3/server.go

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

* ruduce the size of responseWriter

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

http3/server.go

@@ -599,6 +599,9 @@ func (s *Server) handleRequest(conn quic.Connection, str quic.Stream, decoder *q
 	ctx = context.WithValue(ctx, http.LocalAddrContextKey, conn.LocalAddr())
 	req = req.WithContext(ctx)
 	r := newResponseWriter(str, conn, s.logger)
+	if req.Method == http.MethodHead {
+		r.isHead = true
+	}
 	handler := s.Handler
 	if handler == nil {
 		handler = http.DefaultServeMux