Prepend leading slash in ResourceUrlProvider

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

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceUrlProvider.java

@@ -179,6 +179,9 @@ public class ResourceUrlProvider implements ApplicationListener<ContextRefreshed
 			pathWithinMapping = getPathHelper().getLookupPathForRequest(request);
 		}
 
+		// When extracted with PathMatcher, pathWithinMapping won't have leading slash
+		pathWithinMapping = (pathWithinMapping.charAt(0) == '/' ? pathWithinMapping : "/" + pathWithinMapping);
+
 		int index = getPathHelper().getRequestUri(request).indexOf(pathWithinMapping);
 		Assert.state(index != -1, "Failed to determine lookup path: " + requestUrl);