fix node

kindle/amz.py

@@ -58,6 +58,7 @@ def lookup(book):
             if product.languages:
                 book.languages = list(product.languages)
 
+            book.nodes = []
             for browse_node in product.browse_nodes:
                 node = Node()
                 book.nodes.append(node)

kindle/book.py

@@ -28,7 +28,7 @@ class Book:
     publication_date = None
     small_image_url = None
     languages = None
-    nodes = []
+    nodes = None
 
     def json(self):
         return json.dumps(self, default=lambda o: o.__dict__, indent=2, ensure_ascii=False, sort_keys=True)