quickrun buffer is losing &nobuflisted after being closed

[tomushkin]

Steps to reproduce:

1. Load plugin in fresh Vim

$ vim -Nu NONE -c "run plugin/quickrun.vim"

2. Run quickrun and observe listed buffers:

:QuickRun sh -src date
:ls
  1 %a   "[No Name]"                    line 1

3. Close the quickrun window and repeat 2.

<C-w>o
:QuickRun sh -src date
:ls
  1 %a   "[No Name]"                    line 1
  2 #a   "quickrun://output"            line 0