add default value to models

lib/src/models/album.dart

@@ -27,7 +27,7 @@ class Album {
         mediumType = mediumType,
         newest = newest,
         name = json['name'],
-        count = json['count'];
+        count = json['count'] ?? 0;
 
   /// list media in the album.
   ///

lib/src/models/media_page.dart

@@ -19,7 +19,7 @@ class MediaPage {
 
   /// Creates a range of media from platform channel protocol.
   MediaPage.fromJson(this.album, dynamic json)
-      : start = json['start'],
+      : start = json['start'] ?? 0,
         items = json['items'].map<Medium>((x) => Medium.fromJson(x)).toList();
 
   /// Gets the next page of media in the album.

lib/src/models/medium.dart

@@ -49,7 +49,7 @@ class Medium {
     this.width,
     this.height,
     this.size,
-    this.orientation = 0,
+    this.orientation,
     this.mimeType,
     this.duration = 0,
     this.creationDate,