Fix setting exit_on_escape atribute on on_keyboard method (#1967)
Fixes issue with the app terminating on esc even if exit_on_escape is set to '0'
Value could be a string already as set by the user, so it's best if we just use the getboolean function directly
Merge branch 'master' of https://github.com/kivy/kivy into 1967-exception-exit_on_escape
Merge pull request #1970 from mihaineacsu/1967-exception-exit_on_escape

Fix setting exit_on_escape atribute on on_keyboard method