Fix issue w/ use of UrlPathHelper's urlDecode property

Before this change the getPathWithinServletMapping method of
UrlPathHelper could not work properly when a default servlet mapping
(i.e. "/") was used in combination with urlDecode=false. The fact that
the getServletPath() method of HttpServletRequest always returns a
decoded path was getting in the way.

Although there is no way to check Servlet mappings through the Servlet
API, this change aims to detect the given scenario and returns the full
path following the context path thus avoiding URL decoding.

Note that the same can be achieved by setting urlDecode=false and
alwaysUseFullPath=true. However this change ensures that urlDecode
works properly without having to know that.

Issue: SPR-11101