Fix check for secure socket in SMTP extensions (Fixes #1336)

[Jimver]

This PR fixes the check if the SMTP connection uses a secure socket. Before it directly checks the socket field, however this is actually an InternetMessageIO object which inherits from BufferedIO, so you need to access the io field from BufferedIO to actually access the 'real' SSL socket.

This also means we finally see the padlock next to the sent status!

Before:

After:

This fixes #1336