Fix for incompatibility between Sequel and Arel in Rails 3.2.8.

Fixes the issue introduced by commit rails/rails@a1c05dd8b9bd3623289d3aa73dda2943d620cc34 and discussed in issue rails/arel#133.

It's not the ideal solution, but it's only three lines and seems to do the trick.

(And yes, there are apps that use both Sequel and ActiveRecord.)