Fix polymorphic has_many

Fix polymorphic has_many class name to correspont to builder class name

class Product; has_many :addresses, as: 'adressable'; end

class SuperProduct < Product; end;

super_product.addresses.build #=> addressable_type: "Product"

super_product.addresses # => generates selector
"addressable_type"=>"SuperProduct"}

after fix

SuperProduct.addresses # => generates selector
"addressable_type"=>"Product"}