[SPARK-12249][SQL] JDBC non-equality comparison operator not pushed down.

https://issues.apache.org/jira/browse/SPARK-12249
Currently `!=` operator is not pushed down correctly.
I simply added a case for this.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #10233 from HyukjinKwon/SPARK-12249.

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

@@ -281,6 +281,7 @@ private[sql] class JDBCRDD(
    */
   private def compileFilter(f: Filter): String = f match {
     case EqualTo(attr, value) => s"$attr = ${compileValue(value)}"
+    case Not(EqualTo(attr, value)) => s"$attr != ${compileValue(value)}"
     case LessThan(attr, value) => s"$attr < ${compileValue(value)}"
     case GreaterThan(attr, value) => s"$attr > ${compileValue(value)}"
     case LessThanOrEqual(attr, value) => s"$attr <= ${compileValue(value)}"

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

@@ -177,9 +177,11 @@ class JDBCSuite extends SparkFunSuite with BeforeAndAfter with SharedSQLContext
 
   test("SELECT * WHERE (simple predicates)") {
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE THEID < 1")).collect().size === 0)
+    assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE THEID != 2")).collect().size === 2)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE THEID = 1")).collect().size === 1)
     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)
   }
 
   test("SELECT * WHERE (quoted strings)") {