Refs #339 Readonly fields get displayed incorrectly in History view

Instead of passing the Version parameters as initial, override them in the object itself
A possible optimization would be 
```python
initial = self.get_revision_form_data(request, obj, version)
for key, value in initial.items():
    try:
        setattr(obj, key, value)
    except ValueError:
        field = model._meta.get_field(key)
        setattr(obj, field.attname, value)
```
It is slightly more expensive if the model has a lot of ForeignKeys, but it avoids the overhead of get_field() for all other field types. Would you prefer this?