code: func (w *ResponseWriter) WriteHeader(status int) {
  	if w.headersWritten {
  		return
  	}
  
  	w.response.StatusCode = status
  
  	finalHeaders := make(map[string]string)
  	for k, v := range w.headers {
  		if len(v) > 0 {
  			finalHeaders[k] = v[len(v)-1]
  		}
  	}
  
  	if value, ok := finalHeaders["Content-Type"]; !ok || value == "" {
  		finalHeaders["Content-Type"] = "text/plain; charset=utf-8"
  	}
  
  	w.response.Headers = finalHeaders
  
  	w.headersWritten = true
  }
docs: // WriteHeader sends an HTTP response header with status code.
  // If WriteHeader is not called explicitly, the first call to Write
  // will trigger an implicit WriteHeader(http.StatusOK).
  // Thus explicit calls to WriteHeader are mainly used to
  // send error codes.