code: func (h *RequestHeader) Add(key, value string) {
  	k := getHeaderKeyBytes(&h.bufKV, key, h.disableNormalizing)
  	h.h = appendArg(h.h, b2s(k), value, argsHasValue)
  }
docs: // Add adds the given 'key: value' header.
  //
  // Multiple headers with the same key may be added with this function.
  // Use Set for setting a single header for the given key.