Make Canonizer a noop if expected is ""

Canonizer is an as-yet-undocumented utility that redirects requests to a
canonical network location. You'd use it to redirect
http://www.example.com/ to https://example.com/, for example. This
change allows you to pass in the empty string to make canonize a noop.
Merge branch 'master' of github.com:gittip/aspen