Reset the key formatter in the teardown

We were getting intermittent failures when the test for setting
the global key_format setting was running last and then it was
leaking the setup into the next test file.

Author: pixeltrix

test/jbuilder_test.rb

@@ -49,7 +49,7 @@ def empty?
 
 
 class JbuilderTest < ActiveSupport::TestCase
-  setup do
+  teardown do
     Jbuilder.send :class_variable_set, '@@key_formatter', nil
   end