[SPARK-19137][SQL] Fix withSQLConf to reset OptionalConfigEntry c…

…orrectly

## What changes were proposed in this pull request?

`DataStreamReaderWriterSuite` makes test files in source folder like the followings. Interestingly, the root cause is `withSQLConf` fails to reset `OptionalConfigEntry` correctly. In other words, it resets the config into `Some(undefined)`.

```bash
$ git status
Untracked files:
  (use "git add <file>..." to include in what will be committed)

        sql/core/%253Cundefined%253E/
        sql/core/%3Cundefined%3E/
```

## How was this patch tested?

Manual.
```
build/sbt "project sql" test
git status
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes apache#16522 from dongjoon-hyun/SPARK-19137.

sql/core/src/test/scala/org/apache/spark/sql/test/SQLTestUtils.scala

@@ -94,7 +94,13 @@ private[sql] trait SQLTestUtils
    */
   protected def withSQLConf(pairs: (String, String)*)(f: => Unit): Unit = {
     val (keys, values) = pairs.unzip
-    val currentValues = keys.map(key => Try(spark.conf.get(key)).toOption)
+    val currentValues = keys.map { key =>
+      if (spark.conf.contains(key)) {
+        Some(spark.conf.get(key))
+      } else {
+        None
+      }
+    }
     (keys, values).zipped.foreach(spark.conf.set)
     try f finally {
       keys.zip(currentValues).foreach {