reflect.Values may point to tagged objects with
interface type, e.g. x := reflect.ValueOf(new(interface{})).Elem().
We failed to consider this when implementing Elem.
Also, (reflect.Value).Interface() must do one "unboxing"
when it encounters such tagged objects.
i.e., x.Elem().Interface() and x.Interface() are equivalent
in that case.
Also:
- add example of tagged object with interface type.
- untabify (Label).String docstring.
- added tests.
R=crawshaw
CC=golang-dev
https://golang.org/cl/18020044
7a70c382be