small patch for managers.py

[SkiBY]

Django 1.11.
Without name field in init we have an error: '_TaggableManager' object has no attribute 'name'

class C(models.Model):
...
tags = TaggableManager()
...

def get_fields_with_value(self):
    return ((f, getattr(self, f, None)) for f in self.get_fields())