DocumentReference.create is not defined

[LasseRegin]

It seems like the mocked DocumentReference do not have the create method described in the firestore documentation: https://googleapis.dev/python/firestore/latest/document.html#google.cloud.firestore_v1.document.DocumentReference.create

This results in errors like the following:

> doc_ref = collection.document(doc_id)
> doc_ref.create(doc)
E       AttributeError: 'DocumentReference' object has no attribute 'create'

[ckanaar]

Experiencing the same issue.