HBase key mapping can cause ClassCastException.

This was exposed when looking into CDK-441, where the key deserialization was correctly producing Utf8 objects, but the SpecificData record expected a String. The fix is to copy the field's schema into the key schema for all identity partitioners, but this won't always work. If the record has fields with java-class properties, GenericData will not instantiate those objects correctly to be used to reconstruct entities. This is also another reason why it's a good idea not to allow records in the key.