## What changes were proposed in this pull request?
This PR fixes `pivot(Column)` can accepts `collection.mutable.WrappedArray`.
Note that we return `collection.mutable.WrappedArray` from `ArrayType`, and `Literal.apply` doesn't support this.
We can unwrap the array and use it for type dispatch.
```scala
val df = Seq(
(2, Seq.empty[String]),
(2, Seq("a", "x")),
(3, Seq.empty[String]),
(3, Seq("a", "x"))).toDF("x", "s")
df.groupBy("x").pivot("s").count().show()
```
Before:
```
Unsupported literal type class scala.collection.mutable.WrappedArray$ofRef WrappedArray()
java.lang.RuntimeException: Unsupported literal type class scala.collection.mutable.WrappedArray$ofRef WrappedArray()
at org.apache.spark.sql.catalyst.expressions.Literal$.apply(literals.scala:80)
at org.apache.spark.sql.RelationalGroupedDataset.$anonfun$pivot$2(RelationalGroupedDataset.scala:427)
at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:237)
at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36)
at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:39)
at scala.collection.TraversableLike.map(TraversableLike.scala:237)
at scala.collection.TraversableLike.map$(TraversableLike.scala:230)
at scala.collection.AbstractTraversable.map(Traversable.scala:108)
at org.apache.spark.sql.RelationalGroupedDataset.pivot(RelationalGroupedDataset.scala:425)
at org.apache.spark.sql.RelationalGroupedDataset.pivot(RelationalGroupedDataset.scala:406)
at org.apache.spark.sql.RelationalGroupedDataset.pivot(RelationalGroupedDataset.scala:317)
at org.apache.spark.sql.DataFramePivotSuite.$anonfun$new$1(DataFramePivotSuite.scala:341)
at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
```
After:
```
+---+---+------+
| x| []|[a, x]|
+---+---+------+
| 3| 1| 1|
| 2| 1| 1|
+---+---+------+
```
## How was this patch tested?
Manually tested and unittests were added.
Closes#23349 from HyukjinKwon/SPARK-26403.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
56967b7e28