http: Add UnderlyingResponseWriter function

http/http.go

@@ -77,6 +77,13 @@ func (this WrappedHeader) Set (name, value string) string {
 	return ""
 }
 
+// UnderlyingResponseWriter returns the underlying http.ResponseWriter of a
+// WrappedResponseWriter. This is a separate function because it should not be
+// directly accessible to templates.
+func UnderlyingResponseWriter (writer WrappedResponseWriter) http.ResponseWriter {
+	return writer.responseWriter
+}
+
 var _ http.ResponseWriter = new(HTTPResponseRecorder)
 
 // HTTPResponseRecorder is an http.ResponseWriter that can buffer a response to