[FIX] When sending an email using a template, the system should use the outgoing mail server (and 'from' address) associated to the template

Steps to reproduce:
 - configure 2 'outgoing mail servers': server1 (higher priority) and server2 (lower priority)
 - create an email template associated to server2 and to res.partner
 - open a partner and send an email using the created template

server1 is used instead of server2
As this is a workaround for 7, could you add a comment to explicitely tell people doing the forward port not to forward the workaround ? Thanks !