More straightforward way to get the class name

In https://github.com/swanandp/acts_as_list/pull/123 the method used to get the class name seems quite convoluted. I've done some testing and found calling `self.class` works fine in this scenario. I've added some tests for this too.

Let me know if I've missed something.

Memoising also doesn't seem necessary as the test suite runs just as fast without it.