[SPARK-12314][SQL] isnull operator not pushed down for JDBC datasource.

https://issues.apache.org/jira/browse/SPARK-12314
`IsNull` filter is not being pushed down for JDBC datasource.

It looks it is SQL standard according to [SQL-92](http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt), SQL:1999, [SQL:2003](http://www.wiscorp.com/sql_2003_standard.zip) and [SQL:201x](http://www.wiscorp.com/sql20nn.zip) and I believe most databases support this.

In this PR, I simply added the case for `IsNull` filter to produce a proper filter string.

Author: hyukjinkwon <gurwls223@gmail.com>

This patch had conflicts when merged, resolved by
Committer: Reynold Xin <rxin@databricks.com>

Closes #10286 from HyukjinKwon/SPARK-12314.

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

@@ -286,6 +286,7 @@ private[sql] class JDBCRDD(
     case GreaterThan(attr, value) => s"$attr > ${compileValue(value)}"
     case LessThanOrEqual(attr, value) => s"$attr <= ${compileValue(value)}"
     case GreaterThanOrEqual(attr, value) => s"$attr >= ${compileValue(value)}"
+    case IsNull(attr) => s"$attr IS NULL"
     case _ => null
   }
 

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -182,6 +182,7 @@ class JDBCSuite extends SparkFunSuite with BeforeAndAfter with SharedSQLContext
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME = 'fred'")).collect().size === 1)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME > 'fred'")).collect().size === 2)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME != 'fred'")).collect().size === 2)
+    assert(stripSparkFilter(sql("SELECT * FROM nulltypes WHERE A IS NULL")).collect().size === 1)
   }
 
   test("SELECT * WHERE (quoted strings)") {