Fix validity encoding for cert dates after 2049

Fix validity encoding for cert dates after 2049

During the change from the reflection serializer to asn.xslt generation we lost
the metadata that said that X.509 Time GeneralizedTime values need to ignore
fractional seconds.

That means we're generating fractional seconds when the input DateTimeOffset
has them, which means we violate RFC in our generated certificates.

Commit migrated from https://github.com/dotnet/corefx/commit/8a65f0b14cb3f29272a18b2f6dbe4bebfd4d4177