Choices attribute according to documentation

Hello!,

Added a validation for the choices field attribute. I started looking at this issue because WTForms-peewee raised an 'unpack error' using a field declared as Charfield(choices=('OPTION', 'OPTION')) , according to the peewee documentation this must be specified as (value, label) so i added a ValueError exception is the iterable don't follow this specification.


Also i added a test for this field attribute.

Regards