fix the reply link on the site-admin /messages page

explicitly specify the charset on the content-type header so the
mail gem doesn't do something stupid like assigning an Encoding
object to the charset String.

fixes CNVS-11566

test plan:
 - send a conversation message from user A to user B.
 - open /messages for user B.
 - reply to the notification email via the reply link.
 - the conversation should be updated with the new message.

Change-Id: I7ac3d2f9cb5cd1a14fbccc2b4905a6d36e75df42
Reviewed-on: https://gerrit.instructure.com/32321
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>