[SPARK-19691][SQL][BRANCH-2.1] Fix ClassCastException when calculating percentile of decimal column

[maropu]

What changes were proposed in this pull request?

This is a backport of the two following commits: 93aa427

This pr fixed a class-cast exception below;

scala> spark.range(10).selectExpr("cast (id as decimal) as x").selectExpr("percentile(x, 0.5)").collect()
 java.lang.ClassCastException: org.apache.spark.sql.types.Decimal cannot be cast to java.lang.Number
	at org.apache.spark.sql.catalyst.expressions.aggregate.Percentile.update(Percentile.scala:141)
	at org.apache.spark.sql.catalyst.expressions.aggregate.Percentile.update(Percentile.scala:58)
	at org.apache.spark.sql.catalyst.expressions.aggregate.TypedImperativeAggregate.update(interfaces.scala:514)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$1$$anonfun$applyOrElse$1.apply(AggregationIterator.scala:171)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$1$$anonfun$applyOrElse$1.apply(AggregationIterator.scala:171)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$generateProcessRow$1.apply(AggregationIterator.scala:187)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$generateProcessRow$1.apply(AggregationIterator.scala:181)
	at org.apache.spark.sql.execution.aggregate.ObjectAggregationIterator.processInputs(ObjectAggregationIterator.scala:151)
	at org.apache.spark.sql.execution.aggregate.ObjectAggregationIterator.<init>(ObjectAggregationIterator.scala:78)
	at org.apache.spark.sql.execution.aggregate.ObjectHashAggregateExec$$anonfun$doExecute$1$$anonfun$2.apply(ObjectHashAggregateExec.scala:109)
	at

This fix simply converts catalyst values (i.e., Decimal) into scala ones by using CatalystTypeConverters.

How was this patch tested?

Added a test in DataFrameSuite.

[SparkQA]

Test build #73378 has finished for PR 17046 at commit b8178d5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

@hvanhovell okay, ready to review

[hvanhovell]

LGTM - merging to 2.1. Thanks!

Can you close?

[maropu]

Thanks!