Returns valid lender SteamID if game currently played is borrowed.

        steamID: The users ID
        appid_playing: The game player is currently playing
        format: Return format. None defaults to json. (json, xml, vdf)

def is_playing_shared_game(self, steamID, appid_playing, format=None):
        """Returns valid lender SteamID if game currently played is borrowed.

        steamID: The users ID
        appid_playing: The game player is currently playing
        format: Return format. None defaults to json. (json, xml, vdf)

        """
        parameters = {'steamid' : steamID, 'appid_playing' : appid_playing}
        if format is not None:
            parameters['format'] = format
        url = self.create_request_url(self.interface, 'IsPlayingSharedGame', 1,
            parameters)
        data = self.retrieve_request(url)
        return self.return_data(data, format=format)