Fixed compatibility with RobotFramework v2.8

Removed unnecessary assignment of self.current_index for compatibility with RobotFramework v2.8, in new robot version current_index is a property without setter.
You don't need to take care about None in current_index anymore if this will be true "self.current is self._no_current" it will return None by default.

Also fixed keyword.async_javascripts test for 100% passing rate.