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! 
@themetric thanks a ton for looking into this but why do you think it's necessary to write to filesystem here?

Reading the original issue thread wouldn't the zones controller approach looks better here? Or maybe we could write an anonymous controller on specs just to test this feature. Right now looks fine to me just adding the `respond_with` where necessary