Fix Mailgun Reply-To handling

Summary:
When sending the "Reply-To" header to Mailgun, Phabricator would
previously send two headers for every "Reply-To": "Reply-To[0][email]" and
"Reply-To[0][name]". Instead, explicitly build the header as specified by RFC
2822 and send it to Mailgun pre-baked.

Pretty sure this bug was a cargo-cult from the Sendgrid code, where (apparently)
this actually works.

Test Plan:
Triggered an email from Phabricator, saw that the header was sent
properly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8645