`objects` set on both class and instance with `on_init=True`

With `on_init=True` the `objects` of the selector are set both on the instance **and** the class in the callback.


Code:
```python
class P(param.Parameterized):

s = param.Selector()

@param.depends('s', watch=True, on_init=True)
def update(self):
self.param.s.objects = list('abc')

p = P()

p.param.s.objects

P.param.s.objects
```