Fix mongoengine `ListField(EmbeddedDocumentField('Model'))`

 not converting `'Model'` to `Model` and raising `TypeError: 'str' object is not callable`

eg. 
`users = mongoengine.ListField(mongoengine.EmbeddedDocumentField("User"))`

Full traceback:
```
Traceback (most recent call last):
  File "/env/src/flask-admin/flask_admin/contrib/mongoengine/view.py", line 532, in update_model
    form.populate_obj(model)
  File "/env/local/lib/python2.7/site-packages/wtforms/form.py", line 79, in populate_obj
    field.populate_obj(obj, name)
  File "/env/src/flask-admin/flask_admin/model/fields.py", line 86, in populate_obj
    field.populate_obj(fake_obj, 'data')
  File "/env/src/flask-admin/flask_admin/contrib/mongoengine/fields.py", line 35, in populate_obj
    candidate = self.model()
TypeError: 'str' object is not callable
```