Adds setting "next response" to Gateway object. Allows better debugging (without requiring so many mocks, unless wanted). Added tests to ensure it works, and that it does *not* send canned responses unless in debug mode. Also verifies that a canned response is only set once (to ensure that we don't end up with strange, stateful behavior.)
Changes PayTrace::API::Gateway.set_debug to debug=. Raises a PayTrace::Exceptions::ErrorResponse exception on an error response; this is disableable via PayTrace::API::Gateway.raise_exceptions = false. Modifies the Gateway spec to test this behavior (testing default on, also testing that it can be disabled). Changes Gateway.next_response from a Response object to a raw string (this allows more fine-grained testing.)
Modifies Customer object to allow initialization via both a PayTrace customer id and the user-supplied unique id. Modifies specs accordingly.