UserCreateTag: stop dying when a tag pointing to tree or blob is created + test fixes

I'm working on getting UserCreateTag ported to Go as noted. This is a MR to add more testing for the existing Ruby code before before that lands.

In the course of doing that I discovered that we support creating tags pointing to a tree or blob object, but the Ruby code dies on looking up a method on nil while trying to return a response. I'm fixing that bug and making it return something more sensible.