Fixed xamarin bug #13708: ASP.NET routing constraints should be treated as 'convertible to string' in order to be compat with MS.NET

Daniel Lo Nigro (@Daniel15) found a problem with ASP.NET routing logic by which route values when matched against constraints where not being converted into strings, resulting in an incompatibility with MS.NET's routing implementation.

This pull-req fixes this issue (reported by @Daniel15 as xamarin's #13708).