Update field_mixins.py

When min_length is not specified, ng-minlength is set to None and validation is impossible
Update field_mixins.py

Tried with `        if getattr(self, 'min_length', None):
            self.widget.attrs['ng-minlength'] = self.min_length
        if getattr(self, 'max_length', None):
            self.widget.attrs['ng-maxlength'] = self.max_length`
Merge pull request #100 from adrienbrunet/master

Update field_mixins.py