LP1532236 Evergreen should allow for HTML formatted action trigger emails

LP1532236 Evergreen should allow for HTML formatted action trigger emails

Simple fix for allowing the AT template to drive the email headers instead
of replacing the headers blindly. Kudos to tsbere for the unless idea.

Testing: setup an action trigger for SendEmail. Make sure your AT template includes
an email header (EG: Content-Type: text/html;charset=utf-8)
Setup a hold/circulation that meets the criteria of your action trigger. Check the
resuting email's headers to match what you had put into the template.

Signed-off-by: blake <blake@mobiusconsortium.org>