preserve document_id state when setting notebook source

Author: apeng-stripe

jupyter_ydoc/ynotebook.py

@@ -250,7 +250,7 @@ def set(self, value: Dict) -> None:
             # clear document
             self._ymeta.clear()
             self._ycells.clear()
-            for key in [k for k in self._ystate.keys() if k not in ("dirty", "path")]:
+            for key in [k for k in self._ystate.keys() if k not in ("dirty", "path", "document_id")]:
                 del self._ystate[key]
 
             # initialize document