add missing respond_with to some API controllers and add tests 

spree/spree#4967

- update spec to test custom rabl template
- ~~inject custom view into dummy app~~
- ensure usage of respond_with for OrdersController and InventoryUnitsController
- add respond_with to ProductsController

~~In the spec for testing custom rabl template rendering I actually create a rabl template and place it into the dummy app and then delete if after the test runs.  This is the only clean way I could think of doing it while still testing the full functionality.  Let me know if there's a better way that doesn't involve writing the actual view file~~  thanks!