Fix criteria checkbox rendering if value contains non ASCII characters.

fixes #9 

not sure whether it would be better to fix IQuerystringRegistryReader implementation in plone.app.querystring to encode index value's keys properly, but that might effect some lower level stuff, so i just encoded in template. BTW - the traceback was really crappy. The wrapping template swallowed the inner template traceback, so no senceful hint was displayed what happend at all.

cheers