Fix order email assignment via api checkouts#create

Before a request like this:

    curl http://localhost:9292/api/checkouts -X POST --data "order[email]=check@spreecommerce.com"

would create an order with an empty email. Now it also allows to set an
order email different from the current authenticated user via that same
request.

Initially I tried to assign the email provided using

    `order.email = params[:email]`

but calling `save!` wouldn't persist that email, I have not idea why
yet.