Check that 'statusline' option is correct

statusline of quickfix window will be reset in the ftplugin of
$VIMRUNTIME, therefore check that airline is still active

fixes #1447

plugin/airline.vim

@@ -43,6 +43,7 @@ function! s:on_window_changed()
   " different autocommands.
   let l:key = [bufnr('%'), winnr(), winnr('$'), tabpagenr()]
   if get(g:, 'airline_last_window_changed', []) == l:key
+        \ && &stl is# '%!airline#statusline('.winnr().')'
     return
   endif
   let g:airline_last_window_changed = l:key