Fix obj versioning w/non-ASCII container name

If you create a container with a non-ASCII name, and then make another
container with X-Versions-Location: first-cøntåîner, *and* you're
serializing stuff in memcache as json (the default), when the proxy
tries to make a versioned object, it will crash.

The fix is to make sure that get_container_info() always returns strs,
not unicodes.

The long-term fix would be to get rid of simplejson entirely, as its
decoder can't make up its mind whether JSON strings should be Python
strs or unicodes, and that makes it really really easy to write bugs
like this.

Change-Id: Ib20ea5fb884484a4246d7a21a9f1e2ffd82eb04f