bundlespec: fix the "streamv2" and "streamv3-exp" variant In c4aab3661f25, we broken this feature by adding unicode instead of bytes to the dictionary. On the other hand, this feature was never tested, so augment the tests to tests this.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}