Respect custom JoinAssociation class.

So, this is now both Rails 4.1 friendly and Thinking Sphinx + polymorphic association friendly. Albeit with the hackish approach of replacing constants - but I've not found any other way that doesn't involve copying a bunch of internal ActiveRecord code (which I really don't want to do - I'm tying myself to the implementation more closely than I'd like already).
Variable doesn't give us anything - and can become one line.