DATAJPA-527 - Improved handling of CrudRepository.exists(ID) entities with complex composite id.

We now delegate the exists(..) check to findOne(..) in SimpleJpaRepository for entities that have a complex composite id via @IdClass. Previously we tried to generate a string based count query in that case, which didn't work since the parameter types for the IdClass attributes didn't match the values returned by entityInformation.getCompositeIdAttributeValue(..).

Original pull request: #95.