[SPARK-16791][SQL] cast struct with timestamp field fails

## What changes were proposed in this pull request?
a failing test case + fix to SPARK-16791 (https://issues.apache.org/jira/browse/SPARK-16791)

## How was this patch tested?
added a failing test case to CastSuit, then fixed the Cast code and rerun the entire CastSuit

Author: eyal farago <eyal farago>
Author: Eyal Farago <eyal.farago@actimize.com>

Closes #14400 from eyalfa/SPARK-16791_cast_struct_with_timestamp_field_fails.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala

@@ -416,7 +416,7 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
   }
 
   private[this] def cast(from: DataType, to: DataType): Any => Any = to match {
-    case dt if dt == child.dataType => identity[Any]
+    case dt if dt == from => identity[Any]
     case StringType => castToString(from)
     case BinaryType => castToBinary(from)
     case DateType => castToDate(from)

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuite.scala

@@ -727,6 +727,16 @@ class CastSuite extends SparkFunSuite with ExpressionEvalHelper {
     }
   }
 
+  test("cast struct with a timestamp field") {
+    val originalSchema = new StructType().add("tsField", TimestampType, nullable = false)
+    // nine out of ten times I'm casting a struct, it's to normalize its fields nullability
+    val targetSchema = new StructType().add("tsField", TimestampType, nullable = true)
+
+    val inp = Literal.create(InternalRow(0L), originalSchema)
+    val expected = InternalRow(0L)
+    checkEvaluation(cast(inp, targetSchema), expected)
+  }
+
   test("complex casting") {
     val complex = Literal.create(
       Row(