Uniform code and fix for test friendliness (mocha)
Uniform code and fix for test friendliness (mocha)

- replace let with const
- check existence of `RNI18n` when accessing it's properties
- remove unnecessary code
AlexanderZaytsev/react-native-i18n,AlexanderZaytsev/react-native-i18n,AlexanderZaytsev/react-native-i18n,AlexanderZaytsev/react-native-i18n