[SPARK-7946] [MLLIB] DecayFactor wrongly set in StreamingKMeans

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #6497 from MechCoder/spark-7946 and squashes the following commits:

2fdd0a3 [MechCoder] Add non-regression test
8c988c6 [MechCoder] [SPARK-7946] DecayFactor wrongly set in StreamingKMeans

mllib/src/main/scala/org/apache/spark/mllib/clustering/StreamingKMeans.scala

@@ -178,7 +178,7 @@ class StreamingKMeans(
 
   /** Set the decay factor directly (for forgetful algorithms). */
   def setDecayFactor(a: Double): this.type = {
-    this.decayFactor = decayFactor
+    this.decayFactor = a
     this
   }
 

mllib/src/test/scala/org/apache/spark/mllib/clustering/StreamingKMeansSuite.scala

@@ -133,6 +133,13 @@ class StreamingKMeansSuite extends FunSuite with TestSuiteBase {
     assert(math.abs(c1) ~== 0.8 absTol 0.6)
   }
 
+  test("SPARK-7946 setDecayFactor") {
+    val kMeans = new StreamingKMeans()
+    assert(kMeans.decayFactor === 1.0)
+    kMeans.setDecayFactor(2.0)
+    assert(kMeans.decayFactor === 2.0)
+  }
+
   def StreamingKMeansDataGenerator(
       numPoints: Int,
       numBatches: Int,