Simplify example/handlers.py AuthHandler._to_user_models_attrs

Here's how I did it.

```
def _to_user_model_attrs(self, data, attrs_map):
  """Get the needed information from the provider dataset."""
  user_attrs = {}
  for k, v in attrs_map.iteritems():
    attr = (v, data.get(k)) if isinstance(v, str) else v(data.get(k))
    user_attrs.setdefault(*attr)

  return user_attrs
```
