[SPARK-20367] Properly unescape column names of partitioning columns parsed from paths.

## What changes were proposed in this pull request?

When infering partitioning schema from paths, the column in parsePartitionColumn should be unescaped with unescapePathName, just like it is being done in e.g. parsePathFragmentAsSeq.

## How was this patch tested?

Added a test to FileIndexSuite.

Author: Juliusz Sompolski <julek@databricks.com>

Closes #17703 from juliuszsompolski/SPARK-20367.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningUtils.scala

@@ -243,7 +243,7 @@ object PartitioningUtils {
     if (equalSignIndex == -1) {
       None
     } else {
-      val columnName = columnSpec.take(equalSignIndex)
+      val columnName = unescapePathName(columnSpec.take(equalSignIndex))
       assert(columnName.nonEmpty, s"Empty partition column name in '$columnSpec'")
 
       val rawColumnValue = columnSpec.drop(equalSignIndex + 1)

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileIndexSuite.scala

@@ -27,6 +27,7 @@ import org.apache.hadoop.fs.{FileStatus, Path, RawLocalFileSystem}
 
 import org.apache.spark.metrics.source.HiveCatalogMetrics
 import org.apache.spark.sql.catalyst.util._
+import org.apache.spark.sql.functions.col
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.test.SharedSQLContext
 import org.apache.spark.util.{KnownSizeEstimation, SizeEstimator}
@@ -236,6 +237,17 @@ class FileIndexSuite extends SharedSQLContext {
     val fileStatusCache = FileStatusCache.getOrCreate(spark)
     fileStatusCache.putLeafFiles(new Path("/tmp", "abc"), files.toArray)
   }
+
+  test("SPARK-20367 - properly unescape column names in inferPartitioning") {
+    withTempPath { path =>
+      val colToUnescape = "Column/#%'?"
+      spark
+        .range(1)
+        .select(col("id").as(colToUnescape), col("id"))
+        .write.partitionBy(colToUnescape).parquet(path.getAbsolutePath)
+      assert(spark.read.parquet(path.getAbsolutePath).schema.exists(_.name == colToUnescape))
+    }
+  }
 }
 
 class FakeParentPathFileSystem extends RawLocalFileSystem {