Map application/json to JSON serializer.

This is the content type we use in the Signal system.

lib/amqp_utils/serialization.rb

@@ -42,7 +42,7 @@ def self.serialize(message)
   end
 
   class Json < Base
-    content_type 'text/json'
+    content_type 'text/json', 'application/json'
 
     def self.deserialize(message)
       ::JSON.parse(message)