code: func (c *Client) FreeSpace(path string) (freeSpace cunits.Bits, err error) {
  	payload := &transmissionFreeSpacePayload{Path: path}
  	var space TransmissionFreeSpace
  	if err = c.rpcCall("free-space", payload, &space); err == nil {
  		if space.Path == path {
  			freeSpace = cunits.ImportInByte(float64(space.Size))
  		} else {
  			err = fmt.Errorf("returned path '%s' does not match with requested path '%s'", space.Path, path)
  		}
  	} else {
  		err = fmt.Errorf("'free-space' rpc method failed: %v", err)
  	}
  	return
  }
docs: /*
  	Free Space
  	https://github.com/transmission/transmission/blob/2.9x/extras/rpc-spec.txt#L618
  */
  // FreeSpace allow to see how much free space is available in a client-specified folder.