Patch serialisation of field.coerce

RemoteTypedChoiceField and RemoteTypedMultipleChoiceField now display the name of the function defined for field.coerce.  

Previously it called the coerce function in resolve_promise generating:

```
to_python() takes exactly 2 arguments (1 given)

django_remote_forms/utils.py in resolve_promise, line 21
```

Note: I think this could be upgraded to something useful or removed.  Perhaps defining a ajax call or validation spec for client side coercion?