Translating not existing key twice should not set Lit::Localization#_is_changed to true

Set is_changed to true only if translated_value is changed.
Do the same for create_version.
Mark localizations as completed only is_changed is changed to true.
Move tests from pure_i18n_compatibility_extended_test.rb to lit_behaviour_test.rb
Merge pull request #25 from Bonias/fixed-localization-is-change-column-behaviour

Translating not existing key twice should not set Lit::Localization#is_changed to true