Fix the bug that using mutable function in 'zipWithIndex'

Author: zsxwing

language-adaptors/rxjava-scala/src/main/scala/rx/lang/scala/Observable.scala

@@ -407,8 +407,7 @@ trait Observable[+T]
    *         their index. Indices start at 0.
    */
   def zipWithIndex: Observable[(T, Int)] = {
-    var n = 0;
-    this.map(x => { val result = (x,n); n += 1; result })
+    zip((0 until Int.MaxValue).toObservable)
   }
 
   /**

language-adaptors/rxjava-scala/src/test/scala/rx/lang/scala/ObservableTest.scala

@@ -168,4 +168,12 @@ class ObservableTests extends JUnitSuite {
     val o: Observable[String] = List[String]().toObservable.tail
     o.toBlockingObservable.toList
   }
+
+  @Test
+  def testZipWithIndex() {
+    val o = List("alice", "bob", "carol").toObservable.zipWithIndex.map(_._2)
+    assertEquals(List(0, 1, 2), o.toBlockingObservable.toList)
+    assertEquals(List(0, 1, 2), o.toBlockingObservable.toList)
+  }
+
 }