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.
Merge pull request #46 from ad-m/patch-1

Add support to model with strings in max_length