handle model name with more than one word

1)
currently using downcase to form the class map, which does not handles model class name with more than one word correctly. this commit changes the classify mapping to use underscore instead

https://github.com/RestPack/restpack_serializer/issues/73

2)
the query was incorrect when the model is using has_many :through

https://github.com/RestPack/restpack_serializer/issues/74