Rename Auth::Private.create_client to create_token to reflect what it actually does and bring it into line with the acceptance test
tiny bit of editorial clean up
Break out HTTP request-making; introduce the concept of a deserialiser to process responses
Begin to introduce fetch-one requests (i.e. requests using an extra identifier in the URL)
Move the acceptable HTTP method check into Endpoint#fetch_response, add #fetch_one
Make the connection between acceptable HTTP methods and Endpoint creation more explicit in the test
Introduce Serialiser for making Post requests
Improve the way post body / extra id-like args are separated in the args for Endpoint.fetch_response
Deal more sensibly with the fact that HTTP responses' bodies are behind the .body method
Bring the client spec and code in line with what'd been done to Endpoint
Add a real deserialiser for Invoice, albeit one which deserialises as little as possible