Added bean validations tests to ControllerMethodInvokerTest (from my previous pull request).

>> turns out there is an array of tests at ControllerMethodInvokerTest.
>> There are a lot of gems, but the validation should be documented properly and improved...

Added bean validations tests to ControllerMethodInvokerTest.

Regarding improvements.
I think ninja.validation.Validation is documented very well. But site documentation doesn't mention how to use fields violation (it mentions only bean violations). May be this should be added to site documentation?