code: func (m NiceMD) Set(key string, value string) NiceMD {
  	k, v := encodeKeyValue(key, value)
  	m[k] = []string{v}
  	return m
  }
docs: // Set sets the given value in a metadata.
  //
  // It works analogously to http.Header.Set, overwriting all previous metadata values.
  //
  // The function is binary-key safe.