Fix adding a group to a custom user with MongoDB

Using MongoDB as the Nuxeo database, POST /api/v1/user/{userName}/group/{groupName} fails at adding a group to a custom user and no error is returned. 

Steps to repro:

1. In Studio, create a custom user schema (cf user-supnxp-37629.xsd)
2. Create a user

   curl -X POST 'http://localhost:8080/nuxeo/api/v1/user?' -H 'Nuxeo-Transaction-Timeout: 3' -H 'X-NXproperties: *' -H 'X-NXRepository: default' -H 'content-type: application/json' -H 'content-type: application/json' -d '{ "entity-type": "user", "id": "fakeId", "properties": { "lastName": "Cruise", "username": "Tom" } }' -u Administrator:<PASSWORD>

3. Add "Administrators" group to the user

   curl -X POST 'http://localhost:8080/nuxeo/api/v1/user/Tom/group/administrators?' -H 'Nuxeo-Transaction-Timeout: 3' -H 'X-NXproperties: *' -H 'X-NXRepository: default' -H 'content-type: application/json' -u Administrator:<PASSWORD>

Outcome:

The group "administrators" is not added to the user

Expected outcome:

The group "administrators" should be added to the user