set kernel cwd to notebook's directory

closes  #4424

IPython/html/services/sessions/handlers.py

@@ -62,7 +62,7 b' class SessionRootHandler(IPythonHandler):'
         if sm.session_exists(name=name, path=path):
             model = sm.get_session(name=name, path=path)
         else:
-            kernel_id = km.start_kernel(cwd=nbm.notebook_dir)
+            kernel_id = km.start_kernel(cwd=nbm.get_os_path(path))
             model = sm.create_session(name=name, path=path, kernel_id=kernel_id, ws_url=self.ws_url)
         location = url_path_join(self.base_kernel_url, 'api', 'sessions', model['id'])
         self.set_header('Location', url_escape(location))