http3: add Date response header if not set (#3952)

* automatically add date header if not already set

* improve comment for Date header

---------

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

http3/response_writer.go

@@ -55,6 +55,12 @@ func (w *responseWriter) WriteHeader(status int) {
 
 	if status < 100 || status >= 200 {
 		w.headerWritten = true
+		// Add Date header.
+		// This is what the standard library does.
+		// Can be disabled by setting the Date header to nil.
+		if _, ok := w.header["Date"]; !ok {
+			w.header.Set("Date", time.Now().UTC().Format(http.TimeFormat))
+		}
 	}
 	w.status = status