When timing metadata changes, ensure metadata.changed fires

This bug was caused by updating the object in place instead of
copying before each change. With the former, metadata.changed does
not signal.

packages/cells/src/widget.ts

@@ -1083,7 +1083,10 @@ export namespace CodeCell {
           if (!value) {
             return;
           }
-          const timingInfo: any = model.metadata.get('execution') || {};
+          const timingInfo: any = Object.assign(
+            {},
+            model.metadata.get('execution')
+          );
           timingInfo[`iopub.${label}`] = value;
           model.metadata.set('execution', timingInfo);
           return true;
@@ -1096,7 +1099,10 @@ export namespace CodeCell {
       model.executionCount = msg.content.execution_count;
       const started = msg.metadata.started as string;
       if (recordTiming && started) {
-        const timingInfo = (model.metadata.get('execution') as any) || {};
+        const timingInfo = Object.assign(
+          {},
+          model.metadata.get('execution') as any
+        );
         if (started) {
           timingInfo['shell.execute_reply.started'] = started;
         }