fix the polymorphic association bug

while you have an Image model mount as imageable to multiple models, say an article with id to be 5, it would generate a cache key like "uniq_key_Image_imageable_id_5", which is wrong as the type is missing.
After the fix, it should be "uniq_key_Image_imageable_id_5,imageable_type_Article".
Merge pull request #10 from kenniz/master

fix the polymorphic association bug