Avoid HandlerMapping attribute in ResourceUrlProvider

The use of the HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE
in ResourceUrlProvider (as a way of saving lookup path determination)
leads to incorrect results. For example when the request is forwarded
the current requestUri may no longer be compariable to the value of the
PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE. Also where the request is mapped
using a pattern, the value of PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE is
not the same as the lookup path.

This change removes the use of the attribute from ResourceUrlProvider
and instead always determines the lookup path when getForRequestUrl
is called.

Issue: SPR-12332
Polish ResourceUrlEncodingFilter
Cache lookup path in ResourceUrlEncodingFilter

Commit https://github.com/spring-projects/spring-framework/commit/2b97d6
introduced a change where the path within the DispatcherServlet is
determined with each call to ResourceUrlProvider.getForRequestUrl.

To avoid repeating that every time a URL is encoded through the
response, we now cache the result of the lookupPath determination in
ResourceUrlEncodingFilter.

Issue: SPR-12332