`Object::as_tag` does not seems to work as expected.

I've been playing with the tag example, the method Object::as_tag always return none unless the tag has a message :

if let Some(tag) = obj.as_tag() {
                print_tag(tag, args);
            } else if let Some(commit) = obj.as_commit() {
                print_commit(commit, name, args);
            } else {
                print_name(name);
            }

• tagging a commit with git tag v0.0.0 and trying to convert the corresponding object to a tag will return None.
• tagging a commit with git tag -a v0.0.0 -m "some message" and trying to convert the corresponding object to a tag will return Some(Tag { name: "v0.0.0", id: oid... })

Is this an expected behavior ? If it is, It might be a good idea to document it. I'd be happy to submit a PR.

Start with examples/tag.rs and the Object::as_tag API, then reproduce the two cases using lightweight and annotated git tags. Check whether the differing results are expected for the underlying object types; done means the behavior is clarified and the relevant API documentation or conversion is updated as appropriate.