Fix JSON caching regression

Disable jQuery's AJAX caching to prevent the browser's
back button from dumping the stream in JSON format.

Includes a test for the jquery_include_tag helper to
ensure that the necessary line of javascript doesn't
go missing again.

closes #3094

app/helpers/application_helper.rb

@@ -48,6 +48,7 @@ module ApplicationHelper
       buf << [ javascript_include_tag('jquery') ]
     end
     buf << [ javascript_include_tag('jquery_ujs') ]
+    buf << [ javascript_tag("jQuery.ajaxSetup({'cache': false});") ]
     buf.join("\n").html_safe
   end
 end

spec/helpers/application_helper_spec.rb

@@ -78,5 +78,9 @@ describe ApplicationHelper do
     it 'inclues jquery_ujs.js' do
       jquery_include_tag.should match(/jquery_ujs\.js/)
     end
+
+    it "disables ajax caching" do
+      jquery_include_tag.should match(/jQuery\.ajaxSetup/)
+    end
   end
 end