Fix tests for jruby

[yajl-ruby](https://github.com/brianmario/yajl-ruby) [doesn't support jruby](https://github.com/brianmario/yajl-ruby/blob/master/.travis.yml). However, [this test](https://github.com/patriciomacadden/sinatra-contrib/blob/master/spec/respond_with_spec.rb#L232) was failing because it was being run on jruby.