Polymorphic Associations with hash parameters

When using comma in a class that has a polymorphic association, and when specifying a field to use with that association, comma fails with a uninitialized constant error. 

For example:

```ruby 
class MyClass < ActiveRecord::Base
  belongs_to :aggregate, :polymorphic => true

  comma do
    id
    aggregate :name => 'Check'
  end
end
```

Will fail with "uninitialized constant MyClass::Aggregate" when the csv file is generated.

This pull request fixes that error.