String constructor for LocalizedValue

This allows you to do: LocalizedValue('thing'), which would result in a LocalizedValue looking
like this:

     {'en': 'thing', 'ar': None}

localized_fields/fields/localized_value.py

@@ -15,6 +15,9 @@ class LocalizedValue:
                 different language.
         """
 
+        if isinstance(keys, str):
+            setattr(self, settings.LANGUAGE_CODE, keys)
+        else:
             for lang_code, _ in settings.LANGUAGES:
                 value = keys.get(lang_code) if keys else None
                 setattr(self, lang_code, value)