Fix / improve TranslatedModelSerializer to_internal_value()
Fix / improve TranslatedModelSerializer to_internal_value()

Simplified the function and add the same time fixed a bug that
possible extra fields in POST/PUT data caused errors.

aapris/linkedevents,aapris/linkedevents,aapris/linkedevents,tuomas777/linkedevents,City-of-Helsinki/linkedevents,tuomas777/linkedevents,City-of-Helsinki/linkedevents,City-of-Helsinki/linkedevents,tuomas777/linkedevents