[SPARK-16034][SQL] Checks the partition columns when calling...

[SPARK-16034][SQL] Checks the partition columns when calling dataFrame.write.mode("append").saveAsTable

## What changes were proposed in this pull request?

`DataFrameWriter` can be used to append data to existing data source tables. It becomes tricky when partition columns used in `DataFrameWriter.partitionBy(columns)` don't match the actual partition columns of the underlying table. This pull request enforces the check so that the partition columns of these two always match.

## How was this patch tested?

Unit test.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13749 from clockfly/SPARK-16034.