rename Mailer#message

in Rails 3, there's another method with the same name

also fix some specs that check the underlying delivery method

Change-Id: I1e318a280420ea5872b7e4dca097a2b68a0a69e7
Reviewed-on: https://gerrit.instructure.com/28742
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>