prevent validators from getting inserted twice

[pawl]

Fixes #1094

When kwargs in convert() is updated with field_args (really self.form_args), only a referenced to mutable objects in self.form_args are copied. So, modifying kwargs can also modify mutable objects in the overridden self.form_args. Validators like Unique will be added to self.form_args once when the edit form is created, then duplicated when the create form is created.

The solution is to create a deepcopy of field_args to prevent the rest of that code from modifying self.form_args.

Also submitted a pull request to wtf-peewee here: coleifer/wtf-peewee#30