Add support to model with strings in max_lenght 
Add support to model with strings in max_lenght 

Django code:
````
  title = models.CharField(max_length=250)
````
and 
````
  title = models.CharField(max_length="250")
````
are equal, but one of them make django-autofixture fail. This fix that.
ad-m/django-autofixture,gregmuellegger/django-autofixture