Add test which shows Kryo buffer size configured in mb is properly supported

This PR adds test which shows that Kryo buffer size configured in mb is supported properly

Author: tedyu <yuzhihong@gmail.com>

Closes #6390 from tedyu/master and squashes the following commits:

c51ea64 [tedyu] Fix KryoSerializer creation
f12ee04 [tedyu] Correct conf variable name in test
642de51 [tedyu] Drop change in KryoSerializer so that the new test runs
d2fdbc4 [tedyu] Give bufferSizeKb initial value
9a17277 [tedyu] Rewrite bufferSize checking
4739998 [tedyu] Rewrite bufferSize checking
830d0d0 [tedyu] Kryo buffer size configured in mb should be properly supported

core/src/test/scala/org/apache/spark/serializer/KryoSerializerSuite.scala

@@ -62,6 +62,10 @@ class KryoSerializerSuite extends FunSuite with SharedSparkContext {
     val thrown3 = intercept[IllegalArgumentException](new KryoSerializer(conf4).newInstance())
     assert(thrown3.getMessage.contains(kryoBufferProperty))
     assert(!thrown3.getMessage.contains(kryoBufferMaxProperty))
+    val conf5 = conf.clone()
+    conf5.set(kryoBufferProperty, "8m")
+    conf5.set(kryoBufferMaxProperty, "9m")
+    new KryoSerializer(conf5).newInstance()
   }
   
   test("basic types") {