realm and path def enhancements
Tweaks to the optional path segment `/?` and `as`.

The `/` verb is already fairly overloaded so I'm calling this one
`/?`. I also limited its parameter type to `Option[String]`, as it's
too easy to serialize some non-primitive by accident with
`Option[Any]`. We may want to also support `Option[AnyVal]`, but I'll
leave that for later (requires fancy typing).

Also added a test for `/?`