Allow users to not override default mailer configs

Fix from_address logic in mailers, it broke when there were no
mail_method record on the database

Conflicts:
	core/app/mailers/spree/order_mailer.rb
	core/app/mailers/spree/shipment_mailer.rb
	core/app/mailers/spree/test_mailer.rb
	core/app/models/spree/app_configuration.rb