Unable to update credential of new user

[silvinus]

In rxinvoice application we can use the rest endpoint POST /users to create a new user. But when we try to use the endpoint POST /users/{userKey}/credentials we have a mongodb error :

com.mongodb.WriteConcernException: { "serverUsed" : "localhost/127.0.0.1:27017" , "connectionId" : 161 , "err" : "The _id field cannot be changed from {_id: ObjectId('54b3f05f5047b06a5ff2cdac')} to {_id: \"54b3f05f5047b06a5ff2cdac\"}." , "code" : 16836 , "n" : 0 , "ok" : 1.0}

and the credential was not updated.
I think it's because in UserCredentials class the attributes @objectid is missing on the userRef property.

Thanks