fix-root-urls

- APIs that are mounted with a prefix URL, like `http://example.com/api`,
  now work correctly.

- I've left a failing test in place.  The `Heroics.cli_from_schema_url`
  and `Heroics.client_from_schema_url` methods are a bit problematic
  with URL prefixes.

I'm going to go ahead and land this to unblock Mark McGranaghan.