[BUGFIX beta] Properly resolve helpers from {{unbound}}.

Prior to this referencing a helper that would normally need to be looked up with
the `{{unbound}}` helper would not work.

This meant that you could not use helpers that are stored in the container.

This commit fixes that issue.
Merge pull request #4714 from rjackson/properly-resolve-helpers-from-unbound

[BUGFIX beta] Properly resolve helpers from {{unbound}}.