This commit makes sure that the reply-to set for WP e-mails is
preserved when MailPoet is used to send them. Before this change,
MailPoet would ignore whatever was set for the reply-to field for WP
e-mails and use the same address as the one used for the sender instead.
I'm not super happy with the solution proposed here. I couldn't find an
easy way to simple set the reply-to address in the Mailer class and that
is why I'm calling Mailer::init() inside WordPressMailer.
[MAILPOET-3707]
f8388c5497