Fix Redmine/5748 - make defaults promises work in bundles using classes promise

An actually plausible-to-reality use of this feature might look like:
```
vars:
  "a" string => "$(init.one)";
defaults:
  "a" string => "one", if_match_regex => ".*\$(\(.*\)|\{.*\}).*";
```
i.e. if the *a priori* value included an unexpanded variable, discard it in favour of a default.  (... and ISTR @nickanderson recently developed a rather better regex, for detecting that, than the one I just used.)

Not that this is important here.