Support path segment URI var expansion in UrlTag

Before this change UrlTag expanded URI vars and encoded them using
UriUtils.encodePath.

This change makes it possible to expand using
UriUtils.encodePathSegment, which means a "/" is encoded as "%2F".

To expand with path segment semantics, prefix the URI var name "/":

<spring:url value="/url/path/{/var}">
    <spring:param name="var" value="my/Id" />
</spring:url>

Issue: SPR-11401