Merge branch 'dev' of github.com:/us4us/seed into dev

* test/integration/checkout_api_test.rb:

checkout issue found: user a@tz.me can't do checkout which has
a@tz.me only as attandees.
done checkout test. all work correctly. user a do checkout, if attendee
is only a, his wallet doesn't change is correct.
update the checkout create doc
Merge branch 'hbzhou' into dev
1. add checkouts/update integ test. issue found when checkout/search, when
do: checkout = Checkout.find_by(id: checkout_id), the checkout.group_id
is nil. Very strange. Minghe will check.

2. fix typo in checkouts_controller.rb
Merge branch 'hbzhou' into dev
Merge branch 'dev' into minghe_dev
fix the checkout update issue
    1. typoe, group_id ---> group.id, json --> jsono
    2. checkout should be save when update it