Fix path mapping issue in default servlet mode

Prior to this commit, some requests would be wrongly mapped to "/" when:
* requests ended with "//" such as "/foo/bar//"
* the DispatcherServlet was mapped as the default ("/")

In those cases, the HttpServletRequest servlet path was interpreted as
the servlet path, even though the servlet is mappped as the default one.

This commit detects if the current servlet is mapped as the default by
checking the PathInfo value and returns the appropriate path within the
current servlet mapping.

Issue: SPR-12372