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/response_writer.go

@@ -67,9 +67,10 @@ type responseWriter struct {
 	bufferedStr *bufio.Writer
 	buf         []byte
 
-	headerWritten bool
 	contentLen    int64 // if handler set valid Content-Length header
 	numWritten    int64 // bytes written
+	headerWritten bool
+	isHead        bool
 }
 
 var (
@@ -162,6 +163,10 @@ func (w *responseWriter) Write(p []byte) (int, error) {
 		return 0, http.ErrContentLength
 	}
 
+	if w.isHead {
+		return len(p), nil
+	}
+
 	df := &dataFrame{Length: uint64(len(p))}
 	w.buf = w.buf[:0]
 	w.buf = df.Append(w.buf)