Steps to reproduce:

• Create a model with a string field.
• Create and save an instance of the model with the string field populated.
• Add localize: true to the field.

Trying to edit the instance's string field like:
instance.string_field = 'make me a hash'
throws:
NoMethodError: undefined method `merge!' for "example":String

It wants a hash but can't be changed because string case is not handled.