code: func CloneHeader(in http.Header) http.Header {
  	out := make(http.Header, len(in))
  	for key, values := range in {
  		newValues := make([]string, len(values))
  		copy(newValues, values)
  		out[key] = newValues
  	}
  	return out
  }
docs: // CloneHeader creates a deep copy of an http.Header.