title: Moped "invalid collection name"
body: <p>I've recently upgraded mongoid to version 3.0.0 from 2.0 in my rails app.
  When performing an operation which saves an object (either existing or a new one) I'm getting <code>Moped::Errors::OperationFailure: The operation: #&lt;Moped::Protocol::Command</code> saying that there is <code>invalid collection name"</code> .</p>
  
  <p>full_collection_name for the object returns <code>foo_development.$cmd</code>, <code>Mongoid.default_session.collections</code> returns empty array but when i access the database through the mongo client, all the collections are in place.</p>
  
  <p>my mongoid.yml</p>
  
  <p><code>development:
    sessions:
      default:
        database: foo_development
        hosts:
          - localhost:27017</code></p>