Loading json gem for rendering.rb

fixes #1023 (Duplicated issue by mistake sorry :( )

is given with the example from padrino-core/application/rendering.rb :
--Use render { :a => 1, :b => 2, :c => 3 } # => return a json string

Gist of controller/Error: https://gist.github.com/4639694

The controller and rendering work fine if using the .json extension explicitly.