Fixed rare parsing error in DecodeEqualSign, where Convert.ToByte() woul...

This is a fix for an issue I was experiencing when parsing certain emails. It's the same issue that was mentioned on the mailing list a while back: http://sourceforge.net/mailarchive/message.php?msg_id=31144110

It has to do with Convert.ToByte() throwing an ArgumentException instead of a FormatException. It's a simple fix, and I've included a test. It would be great to get this merged into the official repo.

Thanks!
Marty