[SPARK-7694] [MLLIB] Use getOrElse for getting the threshold of LR model

The `toString` method of `LogisticRegressionModel` calls `get` method on an Option (threshold) without a safeguard. In spark-shell, the following code `val model = algorithm.run(data).clearThreshold()` in lbfgs code will fail as `toString `method will be called right after `clearThreshold()` to show the results in the REPL.

Author: Shuo Xiang <shuoxiangpub@gmail.com>

Closes #6224 from coderxiang/getorelse and squashes the following commits:

d5f53c9 [Shuo Xiang] use getOrElse for getting the threshold of LR model
5f109b4 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
c5c5bfe [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
98804c9 [Shuo Xiang] fix bug in topBykey and update test

mllib/src/main/scala/org/apache/spark/mllib/classification/LogisticRegression.scala

@@ -163,7 +163,7 @@ class LogisticRegressionModel (
   override protected def formatVersion: String = "1.0"
 
   override def toString: String = {
-    s"${super.toString}, numClasses = ${numClasses}, threshold = ${threshold.get}"
+    s"${super.toString}, numClasses = ${numClasses}, threshold = ${threshold.getOrElse("None")}"
   }
 }