EIN does not ask to save buffer on exit(again)

[YizhePKU]

Problem description

This seems to be a regression of #734 .

Steps to reproduce the problem

• Run Emacs with emacs -Q -f package-initialize --eval "(setq debug-on-error t)"
• Run M-x ein:login, and connect to the (already running) notebook server
• Click [New Notebook]
• add some text to the first cell
• Do C-x C-c

System info:

("EIN system info"
 :emacs-version
 "GNU Emacs 26.1 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17 Version 10.10.5 (Build 14F2511))
 of 2018-05-31"
 :emacs-bzr-version nil
 :window-system ns
 :emacs-variant nil
 :os
 (:uname
  "Darwin MacBook-Air.local 19.6.0 Darwin Kernel Version 19.6.0: Thu Oct 29 22:56:45 PDT 2020; root:xnu-6153.141.2.2~1/RELEASE_X86_64 x86_64
"
  :lsb-release nil)
 :notebook "WARNING: Package(s) not found: notebook
"
 :ipython "7.19.0
"
 :image-types
 (png gif tiff jpeg xpm xbm pbm)
 :image-types-available
 (png gif tiff jpeg xpm xbm pbm)
 :request
 (:backend curl)
 :ein
 (:version "20210102.1351"
       :source-dir "/Users/name/.emacs.d/elpa/ein-20210102.1351/")
 :lib
 ((:name "websocket"
     :path "~/.emacs.d/elpa/websocket-20210110.17/websocket.elc"
     :featurep t
     :version-var websocket-version
     :version "1.12")
  (:name "anaphora"
     :path "~/.emacs.d/elpa/anaphora-20180618.2200/anaphora.elc"
     :featurep t
     :version-var nil
     :version nil)
  (:name "request"
     :path "~/.emacs.d/elpa/request-20201026.2324/request.elc"
     :featurep t
     :version-var request-version
     :version "0.3.0")
  (:name "deferred"
     :path "~/.emacs.d/elpa/deferred-20170901.1330/deferred.elc"
     :featurep t
     :version-var deferred:version
     :version "0.5.0")
  (:name "polymode"
     :path "~/.emacs.d/elpa/polymode-20200606.1106/polymode.elc"
     :featurep t
     :version-var nil
     :version nil)
  (:name "dash"
     :path "~/.emacs.d/elpa/dash-20210108.1603/dash.elc"
     :featurep t
     :version-var nil
     :version nil)
  (:name "with-editor"
     :path "~/.emacs.d/elpa/with-editor-20201030.1232/with-editor.elc"
     :featurep t
     :version-var nil
     :version nil)))

[dickmao]

Sorry.

I added a regression test this time around in fbf1f71