has_one fallback works when target is not a Singleton

As documented:

> if a Product class `has_one :inventory` calling `Product#inventory` will generate a request on /products/:product_id/inventory.json.

This was implemented in #15, but only implemented and tested for a true Singleton target. It should also work when the target model does not actually include Singleton, e.g. a Post has one Author.

Forgive me for being unfamiliar with the test suite -- I didn't immediately grok why I needed to add the `@post` mock with an explicit `Accept` header. If someone can clarify for me I'll clean up as appropriate.