code: func MaskFrameInPlaceWith(f Frame, m [4]byte) Frame {
  	f.Header.Masked = true
  	f.Header.Mask = m
  	Cipher(f.Payload, m, 0)
  	return f
  }
docs: // MaskFrameInPlaceWith masks frame with given mask and returns frame
  // with masked payload and Mask header's field set.
  // Note that it applies xor cipher to f.Payload without copying, that is, it
  // modifies f.Payload inplace.