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