Don't strip trailing slash if it's escaped in StripSlashes middlware

[nuuls]

This fixes an edge case where a path ending in a trailing encoded slash %2F is incorrectly stripped and rctx.RoutePath is set to the unescaped Path, resulting in routing errors.

The specific edge case in my project is:

• rctx is nil
• rctx.RoutePath is empty
• r.URL.RawPath contains multiple escaped slashes in various places
• rctx.RoutePath is set to the unescaped values contained in r.URL.Path
• The Mux no longer matches the route /link_resolver/{url} because all %2F are now /

A similar issue also exists in RedirectSlashes, I can open a second PR for that as well if you'd like.