VersionedMap.delete() should return true/false

lib/versioned-map.js

@@ -124,6 +124,7 @@ class VersionedMap {
     v[rev] = value;
   }
 
+
   delete(key, rev) {
     if (rev == null)
       rev = this._revision;
@@ -142,7 +143,7 @@ class VersionedMap {
     -- this._sizeDiff[rev];
     v[rev] = NONEXISTENT;
 
-    return old;
+    return true;
   }