[SPARK-5908][SQL] Resolve UdtfsAlias when only single Alias is used

`ResolveUdtfsAlias` in `hiveUdfs` only considers the `HiveGenericUdtf` with multiple alias. When only single alias is used with `HiveGenericUdtf`, the alias is not working.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4692 from viirya/udft_alias and squashes the following commits:

8a3bae4 [Liang-Chi Hsieh] No need to test selected column from DataFrame since DataFrame API is updated.
160a379 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into udft_alias
e6531cc [Liang-Chi Hsieh] Selected column from DataFrame should not re-analyze logical plan.
a45cc2a [Liang-Chi Hsieh] Resolve UdtfsAlias when only single Alias is used.

sql/hive/src/main/scala/org/apache/spark/sql/hive/hiveUdfs.scala

@@ -333,6 +333,8 @@ private[spark] object ResolveUdtfsAlias extends Rule[LogicalPlan] {
       if projectList.exists(_.isInstanceOf[MultiAlias]) && projectList.size != 1 =>
       throw new TreeNodeException(p, "only single Generator supported for SELECT clause")
 
+    case Project(Seq(Alias(udtf @ HiveGenericUdtf(_, _, _), name)), child) =>
+        Generate(udtf.copy(aliasNames = Seq(name)), join = false, outer = false, None, child)
     case Project(Seq(MultiAlias(udtf @ HiveGenericUdtf(_, _, _), names)), child) =>
         Generate(udtf.copy(aliasNames = names), join = false, outer = false, None, child)
   }

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala

@@ -397,6 +397,13 @@ class SQLQuerySuite extends QueryTest {
     dropTempTable("data")
   }
 
+  test("resolve udtf with single alias") {
+    val rdd = sparkContext.makeRDD((1 to 5).map(i => s"""{"a":[$i, ${i+1}]}"""))
+    jsonRDD(rdd).registerTempTable("data")
+    val df = sql("SELECT explode(a) AS val FROM data")
+    val col = df("val")
+  }
+
   test("logical.Project should not be resolved if it contains aggregates or generators") {
     // This test is used to test the fix of SPARK-5875.
     // The original issue was that Project's resolved will be true when it contains