Bugfixed: "Cannot update '__v' and '__v' at the same time" caused by mon...

Bugfixed: "Cannot update '__v' and '__v' at the same time" caused by mongoose when editing a record. 

This could also be achieved by disabling "versionKey" on model definition, but, according to Mongoose documentation (http://mongoosejs.com/docs/guide.html#versionKey), this is not recommended (http://aaronheckmann.tumblr.com/post/48943525537/mongoose-v3-part-1-versioning).