Fail spec: Children should not override parental collection

Mongoid::Sessions when inherited and children override collection should not override parental collection

```
$ rake spec SPEC=spec/mongoid/sessions_spec.rb SPEC_OPTS="-e \"when inherited\""
/home/user/.rvm/rubies/ruby-1.9.3-p392/bin/ruby -S rspec spec/mongoid/sessions_spec.rb
Run options:
  include {:full_description=>/when\ inherited/}
  exclude {:config=>#<Proc:./spec/spec_helper.rb:73>}

Mongoid::Sessions
  when inherited
    and children override collection
      should override children collection
      should not override parental collection (FAILED - 1)

Failures:

  1) Mongoid::Sessions when inherited and children override collection should not override parental collection
     Failure/Error: Actor.collection_name.should eq(:actors)
       
       expected: :actors
            got: :actresses
       
       (compared using ==)
       
       Diff:
       @@ -1,2 +1,2 @@
       -:actors
       +:actresses
       
     # ./spec/mongoid/sessions_spec.rb:1129:in `block (4 levels) in <top (required)>'

Finished in 0.07592 seconds
2 examples, 1 failure

Failed examples:

rspec ./spec/mongoid/sessions_spec.rb:1128 # Mongoid::Sessions when inherited and children override collection should not override parental collection
rake aborted!
/home/user/.rvm/rubies/ruby-1.9.3-p392/bin/ruby -S rspec spec/mongoid/sessions_spec.rb failed
/home/user/.rvm/gems/ruby-1.9.3-p392@mongoid/gems/rspec-core-2.13.1/lib/rspec/core/rake_task.rb:156:in `run_task'
/home/user/.rvm/gems/ruby-1.9.3-p392@mongoid/gems/rspec-core-2.13.1/lib/rspec/core/rake_task.rb:124:in `block (2 levels) in initialize'
/home/user/.rvm/gems/ruby-1.9.3-p392@mongoid/gems/rspec-core-2.13.1/lib/rspec/core/rake_task.rb:122:in `block in initialize'
/home/user/.rvm/gems/ruby-1.9.3-p392@mongoid/bin/ruby_noexec_wrapper:14:in `eval'
/home/user/.rvm/gems/ruby-1.9.3-p392@mongoid/bin/ruby_noexec_wrapper:14:in `<main>'
```