Use 'camelize' instead of 'capitalize' in InstanceKeys#base_class_or_name

Camelize properly handles underscored names. E.G. 'example_model' becomes 'ExampleModel' instead of 'Example_model'.

I ran into this error because I am trying to cache a belongs_to association with an underscore, and simple_cacheable is trying to autoload a non-existent constant.