Move API url test and fix url
Order class api url ends in /orders/ not /order/
Expect POST body to be sent as JSON string
Convert payload to JSON
Separate test content functions and move mock rest client code to test_helper
Orders API url should not end with slash
Expect string response from REST calls
Test Zinc::Order.get gets appropriate order object
Send get request with id appended to URL in Zinc::Order.get
Improve README