Fixes transition from old modifier syntax  to new one

Following the issues with ring.py add-on these days, I've realised that updating one's templates to do the transition wasn't easy: a person having a template {{ring:my_tag:field}} and a plug-in ring which uses my_tag as a parameter will always generate an exception window to pop-up, making it very difficult to reach the template window (and edit the template).

The proposed modification makes sure that an object of type str is sent to the plug-in (it is anyway the only sort of object the plug-in could ever receive, as far as I can see...), so that:
-templates using the old syntax will not crash (they may not work, but the template window can now be reached and edited)
-it makes both syntaxes {{ring():field}} and {{ring:field}} equivalent, and plug-ins don't have to treat {{ring:field}} as a special case (usually generating AttributeError)