bug(ObservableMap): Avoid no-op MapChangeRecord (#22)

* bug(ObservableMap): Avoid no-op MapChangeRecord

i.e., `notifyChange(new MapChangeRecord(this, 'key', 'someValue', 'someValue'))`.

* Run dartfmt.
diff --git a/lib/src/collections/observable_map.dart b/lib/src/collections/observable_map.dart
index e83664d..f1c9191 100644
--- a/lib/src/collections/observable_map.dart
+++ b/lib/src/collections/observable_map.dart
@@ -107,6 +107,9 @@
 
   @override
   void notifyChange([ChangeRecord change]) {
+    if (change is MapChangeRecord && change.oldValue == change.newValue) {
+      return;
+    }
     _allChanges.notifyChange(change);
   }