`AdaptsTo` trait fires change events even if value not changed

This happens only when the value does not "implement" the interface but only "supports" it.

For example, the script

```
from traits.api import *

class ICow(Interface):
pass

@provides(ICow)
class Cow(HasTraits):
pass

class Bull(HasTraits):
pass

register_factory(factory=lambda adaptee: Cow(), from_protocol=Bull, to_protocol=ICow)

class Moo(HasTraits):
cow = AdaptsTo(ICow)

def _cow_changed(self):
print 'CHANGED', self.cow

cow = Cow()
bull = Bull()
moo = Moo()
moo.cow = cow
moo.cow = cow

moo.cow = bull
moo.cow = bull
```

outputs

```
CHANGED <__main__.Cow object at 0x107259b30>
CHANGED <__main__.Bull object at 0x107259b90>
CHANGED <__main__.Bull object at 0x107259b90>
```

while we expect only one Bull event.