fix add-trailing-slash login in base_url

both leading and trailing slash should be added

notebook/notebookapp.py

@@ -759,7 +759,7 @@ class NotebookApp(JupyterApp):
         value = proposal['value']
         if not value.startswith('/'):
             value = '/' + value
-        elif not value.endswith('/'):
+        if not value.endswith('/'):
             value = value + '/'
         return value