Avoid hard-coded value in test setup and teardown.

actionview/test/template/javascript_helper_test.rb

@@ -12,14 +12,14 @@ def update_details(details)
     yield if block_given?
   end
 
-  def setup
-    super
+  setup do
+    @old_escape_html_entities_in_json = ActiveSupport.escape_html_entities_in_json
     ActiveSupport.escape_html_entities_in_json  = true
     @template = self
   end
 
   def teardown
-    ActiveSupport.escape_html_entities_in_json  = false
+    ActiveSupport.escape_html_entities_in_json = @old_escape_html_entities_in_json
   end
 
   def test_escape_javascript