predicate_alist_match: typecheck for RegExp keys
In XULRunner 5 and up, RegExps can no longer be called as functions.
Fuzzing up the definition of 'predicate' to explicitly allow RegExp
objects in predicate_alist_match seemed a reasonable fix.
Resolves issue350.
http://whereswalden.com/2011/03/06/javascript-change-in-firefox-5-not-4-and-in-other-browsers-regular-expressions-cant-be-called-like-functions/
https://bugzilla.mozilla.org/show_bug.cgi?id=313637
https://bugzilla.mozilla.org/show_bug.cgi?id=61911