[SPARK-15321] Fix bug where Array[Timestamp] cannot be encoded/decoded correctly

## What changes were proposed in this pull request?

Fix `MapObjects.itemAccessorMethod` to handle `TimestampType`. Without this fix, `Array[Timestamp]` cannot be properly encoded or decoded. To reproduce this, in `ExpressionEncoderSuite`, if you add the following test case:

`encodeDecodeTest(Array(Timestamp.valueOf("2016-01-29 10:00:00")), "array of timestamp")
`
... you will see that (without this fix) it fails with the following output:

```
- encode/decode for array of timestamp: [Ljava.sql.Timestamp;fd9ebde *** FAILED ***
  Exception thrown while decoding
  Converted: [0,1000000010,800000001,52a7ccdc36800]
  Schema: value#61615
  root
  -- value: array (nullable = true)
      |-- element: timestamp (containsNull = true)
  Encoder:
  class[value[0]: array<timestamp>] (ExpressionEncoderSuite.scala:312)
```

## How was this patch tested?

Existing tests

Author: Sumedh Mungee <smungee@gmail.com>

Closes #13108 from smungee/fix-itemAccessorMethod.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoderSuite.scala

@@ -115,6 +115,7 @@ class ExpressionEncoderSuite extends PlanTest with AnalysisTest {
   encodeDecodeTest("hello", "string")
   encodeDecodeTest(Date.valueOf("2012-12-23"), "date")
   encodeDecodeTest(Timestamp.valueOf("2016-01-29 10:00:00"), "timestamp")
+  encodeDecodeTest(Array(Timestamp.valueOf("2016-01-29 10:00:00")), "array of timestamp")
   encodeDecodeTest(Array[Byte](13, 21, -23), "binary")
 
   encodeDecodeTest(Seq(31, -123, 4), "seq of int")