Unnecessary modification of the class_name in belongs_to breaks class name

I think that there is no need to modify string in class name ```:class_name => 'Foo'``` in ```belongs_to```. According to the comments it was added for compability with **ActiveRecord** and here **belongs_to** only constantized.

Also it's destructive modification, it brakes class name in cases like:

```ruby
'Library::Base'.to_s.pluralize.classify # => 'Library::Basis
```
Test added.