This fixes two bugs:
1. Subclasses were not properly inheriting their parents' Ransack
aliases in Mongoid because each class defined its very own set of
aliases.
2. In Active Record, Ransack aliases were defined in such a way that the
parent's (and grandparent's, etc.) aliases were overwritten by the
child, meaning that all aliases were ultimately kept on
ActiveRecord::Base. This has the unfortunate effect of enforcing
uniqueness of Ransack alias names across all models rather than per
model. Depending on the load order of models, earlier definitions of
an alias in other models would be clobbered.
Steve Richert