[SPARK-14916][MLLIB] A more friendly tostring for FreqItemset in mllib.fpm

## What changes were proposed in this pull request?

jira: https://issues.apache.org/jira/browse/SPARK-14916
FreqItemset as the result of FPGrowth should have a more friendly toString(), to help users and developers.
sample:
{a, b}: 5
{x, y, z}: 4

## How was this patch tested?

existing unit tests.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #12698 from hhbyyh/freqtos.

mllib/src/main/scala/org/apache/spark/mllib/fpm/FPGrowth.scala

@@ -327,5 +327,9 @@ object FPGrowth {
     def javaItems: java.util.List[Item] = {
       items.toList.asJava
     }
+
+    override def toString: String = {
+      s"${items.mkString("{", ",", "}")}: $freq"
+    }
   }
 }