Fix Spree.translations corruption of I18n cache

Fixes #2711
Mailers now require a from address, so we need to give defaults, using MailMethod.current
Correct MailMethod usage in mail_interceptor, order mailer and shipment mailer specs

Conflicts:
	core/spec/mailers/shipment_mailer_spec.rb
Invoke callbacks in #create and #update

Fixes #2736
Correct filename for diners club credit card icon

Fixes #2726

Conflicts:
	frontend/app/assets/images/credit_cards/icons/diners_club.png