Unintentional white spaces in kryo classes configuration parameters

## What changes were proposed in this pull request?

Pruned off white spaces present in the user provided comma separated list of classes for **spark.kryo.classesToRegister** and **spark.kryo.registrator**.

## How was this patch tested?

Manual tests

Author: Victor Chima <blazy2k9@gmail.com>

Closes #12701 from blazy2k9/master.

core/src/main/scala/org/apache/spark/SparkConf.scala

@@ -191,7 +191,8 @@ class SparkConf(loadDefaults: Boolean) extends Cloneable with Logging {
    */
   def registerKryoClasses(classes: Array[Class[_]]): SparkConf = {
     val allClassNames = new LinkedHashSet[String]()
-    allClassNames ++= get("spark.kryo.classesToRegister", "").split(',').filter(!_.isEmpty)
+    allClassNames ++= get("spark.kryo.classesToRegister", "").split(',').map(_.trim)
+      .filter(!_.isEmpty)
     allClassNames ++= classes.map(_.getName)
 
     set("spark.kryo.classesToRegister", allClassNames.mkString(","))

core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala

@@ -71,10 +71,10 @@ class KryoSerializer(conf: SparkConf)
   private val referenceTracking = conf.getBoolean("spark.kryo.referenceTracking", true)
   private val registrationRequired = conf.getBoolean("spark.kryo.registrationRequired", false)
   private val userRegistrators = conf.get("spark.kryo.registrator", "")
-    .split(',')
+    .split(',').map(_.trim)
     .filter(!_.isEmpty)
   private val classesToRegister = conf.get("spark.kryo.classesToRegister", "")
-    .split(',')
+    .split(',').map(_.trim)
     .filter(!_.isEmpty)
 
   private val avroSchemas = conf.getAvroSchema