Patch exists? where scopes are used

AcitveRecord 3 will delegate exists? to the Relation rather than
ActiveRecord::Base if it is called on a scope, meaning that
SomeModel.some_scope.exists? will run a query on the master
Correct method name in test
Test Scope.exists? on Rails 2 and 3
Merge pull request #4 from zendesk/sean/patch_ar3_exists

Patch ActiveRecord 3 `exists?` with scopes