Fixes bug with foreign keys to one-to-one fields

django-simple-history throws an exception when it finds a foreign key to a one-to-one field.

For example, the code below will raise an assertion error saying that a 'NoneType' object has no attribute 'to':

```python
from django.contrib.auth.models import User
from django.db import models
from simple_history.models import HistoricalRecords

class AdminUser(User):
    #The primary key of this model is a OneToOne field to User
    pass

class UserProfile(models.Model)
    user = models.FoerignKey(AdminUser)
    history = HistoricalRecords() 
```

This fixes the issue.