code: func (h *ResponseHeader) Set(key, value string) {
  	initHeaderKV(&h.bufKV, key, value, h.disableNormalizing)
  	h.SetCanonical(h.bufKV.key, h.bufKV.value)
  }
docs: // Set sets the given 'key: value' header.
  //
  // Use Add for setting multiple header values under the same key.