Honour "sendPasswordByTalk" property in mail shares

When a password was set for a mail share an e-mail was sent to the
recipient with the password. Now the e-mail is no longer sent if the
password is meant to be sent by Talk.

However, before the e-mail was not sent when the share was updated but
the password was not changed. Now an e-mail is sent in that case too if
switching from a password sent by Talk to a password sent by mail.

On the other hand, when switching from a password sent by mail to a
password sent by Talk it is mandatory to change the password; otherwise
the recipient would already have access to the share without having to
call the sharer to verify her identity.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>