Fix knownSize overrides

Author: julienrf

src/main/scala/scala/collection/mutable/MultiDict.scala

@@ -21,6 +21,7 @@ class MultiDict[K, V] private (elems: Map[K, Set[V]])
   override def empty: MultiDict[K, V] = multiDictFactory.empty
   override def withFilter(p: ((K, V)) => Boolean): MultiDictOps.WithFilter[K, V, Iterable, MultiDict] =
     new MultiDictOps.WithFilter(this, p)
+  override def knownSize = -1
 
   def sets: collection.Map[K, collection.Set[V]] = elems
 

src/main/scala/scala/collection/mutable/SortedMultiDict.scala

@@ -15,6 +15,8 @@ class SortedMultiDict[K, V] private (elems: SortedMap[K, Set[V]])(implicit val o
     with Growable[(K, V)]
     with Shrinkable[(K, V)] {
 
+  override def knownSize = -1
+
   def sets: collection.SortedMap[K, collection.Set[V]] = elems
 
   override def sortedMultiDictFactory: SortedMapFactory[SortedMultiDict] = SortedMultiDict