The getForRequestUrl method of ResourceUrlProvider uses the
HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE attribute to
determine the relevant portion of the resource URL path.
However there are cases when that attribute may not have a leading
(e.g. when the current URL was matched to a prefix-based pattern
and hence extracted via PathMatcher#extractPathWithinPattern), which
interferes with the matching of resource URL paths to patterns.
This change ensures a leading slash is present
Issue: SPR-12281
24d77f3272