[SPARK-20496][SS] Bug in KafkaWriter Looks at Unanalyzed Plans

## What changes were proposed in this pull request?

We didn't enforce analyzed plans in Spark 2.1 when writing out to Kafka.

## How was this patch tested?

New unit test.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Bill Chambers <bill@databricks.com>

Closes #17804 from anabranch/SPARK-20496-2.