Fix mongoid spec

`mongoid_spec.rb` needs a correct configuration to run, or you'll see this error:

```
  1) OmniAuth::Identity::Models::Mongoid should not use STI rules for its collection name
     Failure/Error: MongoidTestIdentity.collection.name.should == 'mongoid_test_identities'
     Mongoid::Errors::NoSessionConfig:
       
       Problem:
         No configuration could be found for a session named 'default'.
       Summary:
         When attempting to create the new session, Mongoid could not find a session configuration for the name: 'default'. This is necessary in order to know the host, port, and options needed to connect.
       Resolution:
         Double check your mongoid.yml to make sure under the sessions key that a configuration exists for 'default'. If you have set the configuration programatically, ensure that 'default' exists in the configuration hash.
     # ./spec/omniauth/identity/models/mongoid_spec.rb:16:in `block (2 levels) in <top (required)>'
```