Only serialize the Mailbox property of SmtpCommandExtension if it is non-null
Only serialize the Mailbox property of SmtpCommandExtension if it is non-null

Fixes bug #148

nachocove/MailKit,FubarDevelopment/MailKit,dsbenghe/MailKit,matt2003/MailKit,jstedfast/MailKit,mganss/MailKit,eino-makitalo/MailKit,jamie-dainton/MailKit,sstraus/MailKit