Fix render nothing deprecation warning

`render nothing:` is deprecated. If you want nothing rendered you should
use `head :ok`.

Author: eileencodes

test/caching_test.rb

@@ -104,17 +104,17 @@ def simple_runtime_error
 
   def expire
     expire_action controller: 'action_caching_test', action: 'index'
-    render nothing: true
+    head :ok
   end
 
   def expire_xml
     expire_action controller: 'action_caching_test', action: 'index', format: 'xml'
-    render nothing: true
+    head :ok
   end
 
   def expire_with_url_string
     expire_action url_for(controller: 'action_caching_test', action: 'index')
-    render nothing: true
+    head :ok
   end
 
   def streaming