Don't return "_id" when select()ing, as it's inaccessible anyway

[ktoso]

As highlighted in thread: https://groups.google.com/forum/#!topic/rogue-users/VUQFeK4LSbE

We're querying for the _id, but have no way to access it anyway, when using the select(_.name) => (String)syntax, so we could try not querying for it - which would require using the mongofind()differently, not via mongo'sselect().