Add warning when using ok_json. Closes #23

Author: James A. Rosen

lib/multi_json.rb

@@ -25,6 +25,8 @@ def engine
     ["json/pure", :json_pure]
   ]
 
+  DEFAULT_ENGINE_WARNING = 'Warning: multi_json is using default ok_json engine. Suggested action: require and load an appropriate JSON library.'
+
   # The default engine based on what you currently
   # have loaded and installed. First checks to see
   # if any engines are already loaded, then checks
@@ -42,6 +44,7 @@ def default_engine
       end
     end
 
+    Kernel.warn DEFAULT_ENGINE_WARNING
     :ok_json
   end
 

spec/multi_json_spec.rb

@@ -26,6 +26,11 @@
       it 'defaults to ok_json if no other json implementions are available' do
         MultiJson.default_engine.should == :ok_json
       end
+
+      it 'prints a warning' do
+        Kernel.should_receive(:warn).with(/warning/i)
+        MultiJson.default_engine
+      end
     end
 
     it 'defaults to the best available gem' do