fixup! adds groupUntilChanged extension method on scala.collection.Iterator

While considering the comments from the first review, I discovered
that `readHead` was duplicating `Iterator#nextOption`

Author: jeantil

src/main/scala/scala/collection/decorators/IteratorDecorator.scala

@@ -95,24 +95,22 @@ class IteratorDecorator[A](val `this`: Iterator[A]) extends AnyVal {
     */
   def groupUntilChanged[K](f: A => K): Iterator[Iterable[A]] =
     new AbstractIterator[Seq[A]] {
-      private var hd: Option[A] = readHead()
+      private var hd: Option[A] = `this`.nextOption()
       override def hasNext: Boolean = hd.isDefined
 
       override def next(): Seq[A] = {
         hd match {
           case None => Iterator.empty.next()
           case Some(head) =>
             val key = f(head)
-            hd = readHead()
+            hd = `this`.nextOption()
             var seq = mutable.Buffer(head)
             while (hd.exists(el => f(el) == key)) {
               seq = seq ++ hd
-              hd = readHead()
+              hd = `this`.nextOption()
             }
             seq.toVector
         }
       }
-
-      private def readHead() = if (`this`.hasNext) Some(`this`.next) else None
     }
 }