select_class with slist doesn't work in bundle common, looks like evaluation bug.

Consider the attached example:

1. /var/cfengine/bin/cf-agent -K -f ./select_class.cf
   R: Bundle common test_common: class_selected => $(test_common.class_selected)
   R: Bundle agent test_agent: class_selected => agent_1

1. /var/cfengine/bin/cf-agent -K -f ./select_class.cf -v

[...]

verbose: BEGIN initial soft classes:
verbose: C: added soft class _common_classes
verbose: C: added soft class common_class_selected
verbose: END initial soft classes
verbose: ----------------------------------------------------------------

[...]

So, for bundle common it seems the @(common_classes) is auto canonified (_common_classes) instead of being treated as an slist.
This seems to be working in bundle agent as the selection is made correctly.