context.go
func (w contextWriter) Write(p []byte) (int, error) {
	w.context.written = true
	return w.ResponseWriter.Write(p)
}
func (w contextWriter) Write(p []byte) (int, error) {
	w.context.written = true
	return w.ResponseWriter.Write(p)
}
// Write sets the context's written flag before writing the response.