working route tests for spree

* working route tests for 2 API order endpoints, /mine and /apply_coupon_code
* hopefully useful to others wanting to write route tests and improves coverage of spree api

Been having a really hard time writing route tests for Spree...spent a lot of time Googling/Stack-Overflowing and finally found something that works that lets me test routes. The original cause was having /api/orders/mine end up in the orders#show?id=mine incorrectly. The issue was in our app, not API, but not having a test made things really hard to diagnose.

Sending this commit to Spree as a future template for letting other's write route tests. If not accepted, I'll just blog about it.