[Django] mixer ignores registered factories when default values present

It looks like `mixer` completely ignores any registered factories when default values are defined on the model

### Steps to Reproduce
```python
class MyModel(models.Model):
text = models.TextField(default="default")

mixer.register(MyModel, text="non-default")
assert mixer.blend(MyModel).text == "non-default" # this raises :/
```

Start by reproducing the Django model and registration example from the issue, then trace the Django blending path where model defaults are handled. Confirm that a registered factory value takes precedence over the model default, and add or update coverage for this case.

Written by the indexing model from the issue text.