[SPARK-21446][SQL] Fix setAutoCommit never executed

## What changes were proposed in this pull request?
JIRA Issue: https://issues.apache.org/jira/browse/SPARK-21446
options.asConnectionProperties can not have fetchsize，because fetchsize belongs to Spark-only options, and Spark-only options have been excluded in connection properities.
So change properties of beforeFetch from  options.asConnectionProperties.asScala.toMap to options.asProperties.asScala.toMap

## How was this patch tested?

Author: DFFuture <albert.zhang23@gmail.com>

Closes apache#18665 from DFFuture/sparksql_pg.

(cherry picked from commit c972918)
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -286,7 +286,7 @@ private[jdbc] class JDBCRDD(
     conn = getConnection()
     val dialect = JdbcDialects.get(url)
     import scala.collection.JavaConverters._
-    dialect.beforeFetch(conn, options.asConnectionProperties.asScala.toMap)
+    dialect.beforeFetch(conn, options.asProperties.asScala.toMap)
 
     // H2's JDBC driver does not support the setSchema() method.  We pass a
     // fully-qualified table name in the SELECT statement.  I don't know how to