Support partially specified writes from case classes

Currently, we cannot use `rdd.saveToCassandra` on an` RDD[CaseClass]` if CaseClass does not contain all of the fields from the target table. This will currently result in the Error:

This can be limiting if the table has had columns added to it from an external process. If this is the case, a job that writes to the table will automatically fail. This PR proposes a resolution for this issue by making the failure case more permissive (if all of the first n fields of the target table are matched by the case class, then we will attempt the write).