Allow $user_data to be filtered before edd_log_user_in()

This would allow the user_data to be filtered just before sending it to edd_log_user_in(), perfect in the case of EDD Auto Register. 

I looked at your suggestion in #1750, but from what I can see, the `edd_insert_user_args` filter in the same function only allows manipulation of the user_args, not user_data. Tested and works beautifully :)