System address is the first user's addresses

In the preferences module, we don't save the user's custom email address
when it matches the system address. This address was considered the last
of all the user's email addresses, which is first, not intuitive, and
second, incorrect when composing a message since the first address is
used by default.

Fixes #3551

SoObjects/SOGo/SOGoUser.m

@@ -317,7 +317,7 @@
   if (!allEmails)
     [self _fetchAllEmails];
 
-  return [allEmails lastObject];
+  return [allEmails objectAtIndex: 0];
 }
 
 - (BOOL) hasEmail: (NSString *) email