SPARK-2869 - Fix tiny bug in JdbcRdd for closing jdbc connection

I inquired on  dev mailing list about the motivation for checking the jdbc statement instead of the connection in the close() logic of JdbcRDD. Ted Yu believes there essentially is none-  it is a simple cut and paste issue. So here is the tiny fix to patch it.

Author: Stephen Boesch <javadba>
Author: Stephen Boesch <javadba@gmail.com>

Closes #1792 from javadba/closejdbc and squashes the following commits:

363be4f [Stephen Boesch] SPARK-2869 - Fix tiny bug in JdbcRdd for closing jdbc connection (reformat with braces)
6518d36 [Stephen Boesch] SPARK-2689 Fix tiny bug in JdbcRdd for closing jdbc connection
3fb23ed [Stephen Boesch] SPARK-2689 Fix potential leak of connection/PreparedStatement in case of error in JdbcRDD
095b2c9 [Stephen Boesch] Fix tiny bug (likely copy and paste error) in closing jdbc connection

(cherry picked from commit 2643e66008cc04074454bb98613c53ba3300e428)
Signed-off-by: Reynold Xin <rxin@apache.org>