Require 'mail' gem in Spree::Core

Fixes #3342
Remove setting of ActiveRecord::Base.include_root_in_json

See this conversation: https://groups.google.com/forum/#!topic/spree-user/D9dZQayC4zs
[api] Remove before_address from CheckoutsController

This was causing an issue where an order couldn't be updated once it was in the 'address' state because address details had not been provided. If these records aren't even built, then an update will succeed just fine.
[api] Remove test checking for now-impossible (see last commit) situation in CheckoutsController