fixes two bugs introduces by ab6118b 

getActualMethod threw an OutOfBoundsException because it wasn't ensured that the java Method instance and the ExecutableRef have the same amount of parameters ([introduced here](https://github.com/INRIA/spoon/commit/ab6118bfd52e18cc5d296a70a89bebb1b222ea58#diff-21aad886cfe9c33552d811cbb87e539eL240). Also fixes a bug that getActualMethod and getActualClass returned null because of using getActualTypeParameters instead of getParameters..

Also this PR refactors the code of the methods and removes a unnecessary variable