objects container shared when check_on_set is False

Should the `objects` container of a `Selector` be shared when `check_on_set` is `False`?

```python
import param

class P(param.Parameterized):
s = param.Selector(default=1, objects=[1, 2], check_on_set=False)

pa = P(s=3)
pb = P(s=4)
p = P()

print(p.param.s.objects)
# [1, 2, 3, 4]
```