Update X509Certificate.cs

SHA 256 Fix for "Unsupported hash for SSL certificate" bug
http://fogbugz.unity3d.com/default.asp?602783_4ddsl9l014uuvquo

No idea if this is the right branch, or even if you accept pull requests.

I haven't tested it, but I assume it works.  It's based off a patch from here https://bugzilla.xamarin.com/show_bug.cgi?id=8788