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! 
@huoxito thanks for your review.  I felt that creating a custom template was more complete and followed what users would do from the documentation but now I tested using the ZonesController method and the coverage is the same.  I feel it's not exactly testing the override functionality but it does cover the template rendering logic.  I'll change the specs soon and let you know.  Thanks!