Make NoShippingRequired available by default.

When all product classes have shipping_required=False the checkout logic
automatically returns the shipping method NoShippingRequired. That shipping
method is not available in the default Repository so the checkout will
crash.

Adding NoShippingRequired allows the checkout to work in that cases without
overriding the Repository class.
Merge branch 'origin/fix/shipping_method_failure' into machtfit

* branch origin/fix/shipping_method_failure:
  Allow shipping_address being 'None' for an order
Merge branch 'add-no-shipping-required-to-default-repository' into machtfit

* branch add-no-shipping-required-to-default-repository:
  Make NoShippingRequired available by default.