subclasses

Does it make sense to allow subclasses to be used as sources. For example

``` python
class C:
def m(self):
return [1, 2, 3]

class D(C):
def m(self):
return super() + [4, 5, 6]

@convert.register(list, C)
def _(n, **kwargs):
return n.m()

odo(D(), list)
```

Currently, this gives `KeyError: `; however, as long as you have a well formed subclass, I think this could be resolved