KeyWrapperComparator throws exception

TopNKeyOperator.KeyWrapperComparator throws exception when a new key and a copied key should be compared.

The current implementation uses the standard object inspectors for all KeyWrapper instances. However when comparing untouched KeyWrappers the key object inspector should be used which can be extracted form Operator.inputObjectInspectors during initialization of the key's ExprNodeEvaluator. 

This can cause a ClassCastException when the comparator is used collections like TreeSet.