Fix polymorphic has_many 

Fix polymorphic has_many class name to correspont to builder class name:

How to reproduce error:

```ruby
class Product; has_many :addresses, as: 'addressable'; end

class SuperProduct < Product; end;

super_product.addresses.build # sets addressable_type: "Product" for address

super_product.addresses #generates selector "addressable_type"=>"SuperProduct"
```

after fix:

super_product.addresses #generates selector "addressable_type"=>"Product"}