fix names of notebooks for download/save

was using notebook.get('name') instead of notebook.metadata.get('name'),
where the name is actually stored.  The result was that all downloaded notebooks were called 'notebook'.

closes  #2227

IPython/frontend/html/notebook/nbmanager.py

@@ -117,7 +117,7 b' class NotebookManager(LoggingConfigurable):'
             # should match the Python in-memory format.
             kwargs['split_lines'] = False
         data = current.writes(nb, format, **kwargs)
-        name = nb.get('name','notebook')
+        name = nb.metadata.get('name','notebook')
         return last_modified, name, data
 
     def read_notebook_object(self, notebook_id):