[SPARK-11474][SQL] change fetchSize to fetchsize

In DefaultDataSource.scala, it has
override def createRelation(
sqlContext: SQLContext,
parameters: Map[String, String]): BaseRelation
The parameters is CaseInsensitiveMap.
After this line
parameters.foreach(kv => properties.setProperty(kv._1, kv._2))
properties is set to all lower case key/value pairs and fetchSize becomes fetchsize.
However, in compute method in JDBCRDD, it has
val fetchSize = properties.getProperty("fetchSize", "0").toInt
so fetchSize value is always 0 and never gets set correctly.

Author: Huaxin Gao <huaxing@oc0558782468.ibm.com>

Closes #9473 from huaxingao/spark-11474.