Fix ein:notebook-close-worksheet

What ein:notebook-close-worksheet did was horribly wrong.
It deleted a worksheet.  Instead, now it properly close worksheet.

The failing test added in the previous commit passes now
(ein:notebook-to-json-after-closing-a-worksheet).

lisp/ein-notebook.el

@@ -215,9 +215,14 @@ Current buffer for these functions is set to the notebook buffer.")
 call notebook destructor `ein:notebook-del'."
   (symbol-macrolet ((worksheets (ein:$notebook-worksheets notebook))
                     (scratchsheets (ein:$notebook-scratchsheets notebook)))
-    (setq worksheets (delq ws worksheets))
-    (setq scratchsheets (delq ws scratchsheets))
-    (unless (or worksheets scratchsheets)
+    (cond
+     ((ein:worksheet-p ws) (ein:worksheet-save-cells ws))
+     (t (setq scratchsheets (delq ws scratchsheets))))
+    (unless (or (ein:filter (lambda (x)
+                              (and (not (eq x ws))
+                                   (ein:worksheet-has-buffer-p x)))
+                            worksheets)
+                scratchsheets)
       (ein:notebook-del notebook))))
 
 

lisp/ein-worksheet.el

@@ -225,6 +225,19 @@ this value."
                         (ein:worksheet-get-cells ws))))
     `((cells . ,(apply #'vector cells)))))
 
+(defmethod ein:worksheet-save-cells ((ws ein:worksheet))
+  "Save cells in worksheet buffer in cache before killing the buffer.
+
+.. warning:: After calling this function, cells in worksheet
+   cannot be used.  Use only just before killing the buffer.
+
+Do nothing when the worksheet WS has no buffer."
+  (when (ein:worksheet-has-buffer-p ws)
+    (let ((cells (ein:worksheet-get-cells ws)))
+      (mapc #'ein:cell-save-text cells)
+      (mapc #'ein:cell-deactivate cells)
+      (oset ws :saved-cells cells))))
+
 
 ;;; Cell indexing, retrieval, etc.