Add index to Spree::User.spree_api_key

Fixes #3884
Updated docstring explaining how to include controller requests test support in rspec.

Fixes #3898
Dont lazy load variants in Shipment `manifest`

Rails 4.0.1 fixes an issue where `default_scope` would not be applied to
`joins` association, see https://github.com/rails/rails/commit/b407839601c8dc7b007a6baddea3696b737c352b
(I liked that bug pretty much myself). Side effect is users upgrading to
Rails 4.0.1 would see errors when displaying orders with deleted variants
Use actual assertions in credit_card_spec
CreditCard#expiry=: Only prefix '20' to year if 2-digit year is specified
Use evaluate_script instead of execute_script

evaluate_script was meant to be used for these kinds of situations.

Fixes #3894