[Minor] [SQL] [SPARK-6729] Minor fix for DriverQuirks get

The function uses .substring(0, X), which will trigger OutOfBoundsException if string length is less than X. A better way to do this is to use startsWith, which won't error out in this case.

Author: Volodymyr Lyubinets <vlyubin@gmail.com>

Closes #5378 from vlyubin/quirks and squashes the following commits:

504e8e0 [Volodymyr Lyubinets] Minor fix for DriverQuirks get

sql/core/src/main/scala/org/apache/spark/sql/jdbc/DriverQuirks.scala

@@ -49,9 +49,9 @@ private[sql] object DriverQuirks {
    * Fetch the DriverQuirks class corresponding to a given database url.
    */
   def get(url: String): DriverQuirks = {
-    if (url.substring(0, 10).equals("jdbc:mysql")) {
+    if (url.startsWith("jdbc:mysql")) {
       new MySQLQuirks()
-    } else if (url.substring(0, 15).equals("jdbc:postgresql")) {
+    } else if (url.startsWith("jdbc:postgresql")) {
       new PostgresQuirks()
     } else {
       new NoQuirks()