Develop a way that verifies if all attributes are visited by the Dot visitor

[mvgijssel]

In #106 lib/arel/extensions/select_statement.rb some additional visit_edge calls were added. Not implementing these calls currently goes unnoticed, as only implemented calls are verified for correctness.

Let's see if there's a way to extend spec/support/visitors.rb:19 to verify if all attributes of an object are actually visited by the Dot visitor (and raise an exception if this is not the case).