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.
Merge pull request #100 from DisruptionCorporation/has-one-for-non-singleton

has_one fallback works when target is not a Singleton