migrate job fails when target table has more columns than source table

At a customer environment, we noticed that when the target table schema had additional columns compared to the source, we would see errors like the following: ``` 25/06/17 10:21:10 ERROR CopyJobSession: Error with PartitionRange -- ThreadID: 115 Processing min: -6271892985061247544 max: -5902958103587056512 java.util.concurrent.CompletionException: com.datastax.oss.driver.api.core.servererrors.InvalidQueryException: Invalid unset value for column email_typ_cde ... ``` If we could auto-account for the mismatched col count, and leave the rest unset, it would allow for success in these scenarios.