(stable) Support "encoding", which is required by PHPMailerLite, in SES adapter

Summary: See https://discourse.phabricator-community.org/t/amazon-ses-adapator-broken-after-upgrade/1121. The adapter class tree is a mess and this property is read by the parent class.

Test Plan: Configured an SES mailer, used bin/mail send-test to reproduce the issue before the patch and observe it working after the patch.

Differential Revision: https://secure.phabricator.com/D19083