Include scratch sheet in ein:notebook-buffer

Now it is possible to have worksheet buffer closed but scratchsheet
buffer opened.  In this case, ein:notebook-live-p should return t
and ein:notebook-buffer should return the scratchsheet buffer.
This is because notebook commands can such as save or rename can be
executed from the scratchsheet buffer.

ein:notebook-to-json-after-closing-a-worksheet is modified also
this test fails without the change in ein:notebook-buffer.

lisp/ein-notebook.el

@@ -231,8 +231,11 @@ call notebook destructor `ein:notebook-del'."
 (defun ein:notebook-buffer (notebook)
   "Return the buffer that is associated with NOTEBOOK."
   ;; FIXME: Find a better way to define notebook buffer! (or remove this func)
-  (loop for ws in (ein:$notebook-worksheets notebook)
-        if (ein:worksheet-buffer ws) return it))
+  (let ((first-buffer
+         (lambda (ws-list)
+           (loop for ws in ws-list if (ein:worksheet-buffer ws) return it))))
+    (or (funcall first-buffer (ein:$notebook-worksheets    notebook))
+        (funcall first-buffer (ein:$notebook-scratchsheets notebook)))))
 
 (defun ein:notebook-buffer-list (notebook)
   "Return the buffers associated with NOTEBOOK's kernel.

tests/test-ein-notebook.el

@@ -729,6 +729,7 @@ defined."
       (should-not (ein:notebook-modified-p notebook))
       ;; Kill a worksheet buffer
       (kill-buffer buffer)
+      (should (ein:notebook-live-p notebook))
       ;; to-json should still work
       (funcall test notebook))))