Test name changed, comments added

Author: Jakub Dubovský

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

@@ -1411,6 +1411,7 @@ object SparkContext extends Logging {
 
   implicit def bytesWritableConverter(): WritableConverter[Array[Byte]] = {
     simpleWritableConverter[Array[Byte], BytesWritable](bw =>
+      // getBytes method returns array which is longer then data to be returned
       Arrays.copyOfRange(bw.getBytes, 0, bw.getLength)
     )
   }

core/src/test/scala/org/apache/spark/SparkContextSuite.scala

@@ -22,7 +22,8 @@ import org.scalatest.FunSuite
 import org.apache.hadoop.io.BytesWritable
 
 class SparkContextSuite extends FunSuite {
-  test("test of writing spark scala test") {
+  //Regression test for SPARK-3121
+  test("BytesWritable implicit conversion is correct") {
     val bytesWritable = new BytesWritable()
     val inputArray = (1 to 10).map(_.toByte).toArray
     bytesWritable.set(inputArray, 0, 10)