Use "user" as default buffer namespace

In my environment, emacs always freezes after auto-complete popup shows. And after seconds it shows error message.

```
Error running timer `ac-quick-help': (error "Sync nREPL request timed out (op complete-doc session 1def0d59-175a-4963-a5e2-1c93f2088e07 ns nil symbol do id 59)")
```

I think the cause is that `nrepl-buffer-ns` is `nil`(see the code changes).
I don't know why `nrepl-buffer-ns` is set to `nil` but in case it is `nil`, ac-cider should pass "user" as default namespace.