Implement allow_transactions feature (#1410)

You have a configuration option`allow_transactions`, but looks like implementation is missing.

Author: revis0r

lib/jsonapi/request.rb

@@ -34,7 +34,7 @@ def transactional?
       when 'index', 'show_related_resource', 'index_related_resources', 'show', 'show_relationship'
         false
       else
-        true
+        JSONAPI.configuration.allow_transactions
       end
     end