strange `post_init` behavior with `on_trait_change`

```
from traits.api import HasStrictTraits, Event, on_trait_change, List, Instance
class Baby(HasStrictTraits):
cry = Event

class Mom(HasStrictTraits):
babies = List(Baby)
@on_trait_change('babies:cry', post_init=True)
def respond(self):
print 'calm down!'

babies = [Baby() for i in xrange(3)]
mom = Mom(babies=babies)
mom.babies[0].cry = True
```

the `Mom` object doesn't respond to `cry` with `post_init=True`, but works when `post_init=False`. This shouldn't affect the `on_trait_change` call.