SLING-4236 Clarify XSSAPI with respect to null and empty input strings

* Include patch by Radu Cotescu (thanks a lot)
* Extend patch to getValid* and filterHtml methods
* Leverage TestCase.assertEquals for result validation
* Add more tests for empty and null input
* Add LongValidationRule 

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1644580 13f79535-47bb-0310-9956-ffa450edef68