Handle the deprecation of render :text

In 4.1 render :text was deprecated and replaced with :plain and :html so
we need to conditional define a method to handle the difference.

Author: pixeltrix

lib/action_controller/caching/actions.rb

@@ -173,11 +173,21 @@ def around(controller)
             body = controller._save_fragment(cache_path.path, @store_options)
           end
 
-          body = controller.render_to_string(plain: body, layout: true) unless cache_layout
+          body = render_to_string(controller, body) unless cache_layout
 
           controller.response_body = body
           controller.content_type = Mime[cache_path.extension || :html]
         end
+
+        if ActionPack::VERSION::STRING < '4.1'
+          def render_to_string(controller, body)
+            controller.render_to_string(text: body, layout: true)
+          end
+        else
+          def render_to_string(controller, body)
+            controller.render_to_string(plain: body, layout: true)
+          end
+        end
       end
 
       class ActionCachePath