Add the ability for custom headers
Add the ability for custom headers

jorgemanrubia/mailgun_rails,rgraff/mailgun_rails,rgraff/mailgun_rails,nicolasmlv/mailgun_rails,jorgemanrubia/mailgun_rails,Healthism/mailgun_rails,Healthism/mailgun_rails,nicolasmlv/mailgun_rails,jorgemanrubia/mailgun_rails,Healthism/mailgun_rails,rgraff/mailgun_rails