[SPARK-16429][SQL] Include `StringType` columns in `describe()`

## What changes were proposed in this pull request?

Currently, Spark `describe` supports `StringType`. However, `describe()` returns a dataset for only all numeric columns. This PR aims to include `StringType` columns in `describe()`, `describe` without argument.

**Background**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe("age", "name").show()
+-------+------------------+-------+
|summary|               age|   name|
+-------+------------------+-------+
|  count|                 2|      3|
|   mean|              24.5|   null|
| stddev|7.7781745930520225|   null|
|    min|                19|   Andy|
|    max|                30|Michael|
+-------+------------------+-------+
```

**Before**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe().show()
+-------+------------------+
|summary|               age|
+-------+------------------+
|  count|                 2|
|   mean|              24.5|
| stddev|7.7781745930520225|
|    min|                19|
|    max|                30|
+-------+------------------+
```

**After**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe().show()
+-------+------------------+-------+
|summary|               age|   name|
+-------+------------------+-------+
|  count|                 2|      3|
|   mean|              24.5|   null|
| stddev|7.7781745930520225|   null|
|    min|                19|   Andy|
|    max|                30|Michael|
+-------+------------------+-------+
```

## How was this patch tested?

Pass the Jenkins with a update testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14095 from dongjoon-hyun/SPARK-16429.

1 files changed, 2 insertions, 2 deletions

diff --git a/R/pkg/inst/tests/testthat/test_sparkSQL.R b/R/pkg/inst/tests/testthat/test_sparkSQL.R
index e2a1da0f1e..fdd6020db9 100644
--- a/R/pkg/inst/tests/testthat/test_sparkSQL.R
+++ b/R/pkg/inst/tests/testthat/test_sparkSQL.R
@@ -1824,11 +1824,11 @@ test_that("describe() and summarize() on a DataFrame", {
   expect_equal(collect(stats)[2, "age"], "24.5")
   expect_equal(collect(stats)[3, "age"], "7.7781745930520225")
   stats <- describe(df)
-  expect_equal(collect(stats)[4, "name"], NULL)
+  expect_equal(collect(stats)[4, "name"], "Andy")
   expect_equal(collect(stats)[5, "age"], "30")
 
   stats2 <- summary(df)
-  expect_equal(collect(stats2)[4, "name"], NULL)
+  expect_equal(collect(stats2)[4, "name"], "Andy")
   expect_equal(collect(stats2)[5, "age"], "30")
 
   # SPARK-16425: SparkR summary() fails on column of type logical