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.

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