[SPARK-8386] [SQL] add write.mode for insertIntoJDBC when the parm overwrite is false

the fix is for jira https://issues.apache.org/jira/browse/SPARK-8386

Author: Huaxin Gao <[email protected]>

Closes #9042 from huaxingao/spark8386.

(cherry picked from commit 7e1308d)
Signed-off-by: Reynold Xin <[email protected]>

Author: huaxingao

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -1625,7 +1625,7 @@ class DataFrame private[sql](
    */
   @deprecated("Use write.jdbc()", "1.4.0")
   def insertIntoJDBC(url: String, table: String, overwrite: Boolean): Unit = {
-    val w = if (overwrite) write.mode(SaveMode.Overwrite) else write
+    val w = if (overwrite) write.mode(SaveMode.Overwrite) else write.mode(SaveMode.Append)
     w.jdbc(url, table, new Properties)
   }