Merge 4ea7d51deac965727d145929eb0e8f812fb334f8 into d1a15347b1e45a8ee816193d3578a93bfd73b70f

When the response has been written, the status code saved in `responseWriter`
should no longer be changed by subsequent calls to `WriteHeader()`.

This fixes for example the logging possibly showing the wrong status code, or
any other middleware function checking the status code.

response_writer_test.go

@@ -84,6 +84,10 @@ func TestResponseWriterWriteHeadersNow(t *testing.T) {
 	writer.size = 10
 	w.WriteHeaderNow()
 	assert.Equal(t, 10, w.Size())
+
+	w.WriteHeader(http.StatusOK)
+	assert.Equal(t, http.StatusMultipleChoices, w.Status())
+	assert.Equal(t, http.StatusMultipleChoices, testWriter.Code)
 }
 
 func TestResponseWriterWrite(t *testing.T) {