v2.1.4: Trace IDs can be string-like

Bugfix: If we put a trace ID into the context, but it's not an actual string
(Stringer, subtype of string), we wouldn't fetch it out.

Instead, if the key is present and is string-like, use it.
If it's present and not string-like, add !BADVALUE- and use it.
It's very confusing if the key is ignored entirely when set.

699bd84

Full Changelog: v2.1.3...v2.1.4