Fix #28 SA ignore obj kwarg

Yes, I know the differences between them.

The process in  BaseForm is the only one function which takes the "obj" kwarg 
https://github.com/wtforms/wtforms/blob/master/wtforms/form.py#L97
NONE of them put "obj"  into self.\_obj 

On the other hand, the unique validator check self.\_obj, which, I wonder if you guys forget about the self._obj
https://github.com/wtforms/wtforms/blob/master/wtforms/ext/sqlalchemy/validators.py#L32