Add unit tests for the Jsonp filter #3

Affects: v.1.0.1

It would be useful to have some unit tests for the JsonpFilter and associated 
classes. I've put some up here if you'd like to take a look:
https://gist.github.com/1282985

Please feel free to use as you see fit. The tests do introduce some new test 
scope dependencies (junit, hamcrest, mockito, spring-test etc) so you might 
prefer to trim these down.

Thanks for contributing a very useful filter.
Cheers, Ash