the primary key of new row was `null`; skip the `idKey` when update

[lneoe]

• the primary key of new row was null when we create a new row like

  var user = User(id: nil, name: "Vapor", email: "mongo@vapor.codes")
  try user.save()

  the user.id will be null. skip the _id and the MongoKitten will fill this key automatically.

• An Exception was raise when update

  "exception" : "The _id field cannot be changed from {_id: ObjectId('5bc5f5570b6ecc7bf24d738b')} to {_id: \"5bc5f5570b6ecc7bf24d738b\"}.",

  In most cases, we will not modify the primary key, so skip it too.

[tanner0101]

Awesome, thanks!