Enable calling Post using a Secured URI

[carpnick]

Needed the ability to call POST with the key attached to the end.

The Use case:
Imagine having a testset we wanted to add a test case to. The way to do this:

Endpoint: = testset/TESTSET_NUMBER//testcases/add
// Defining the JSON PayLoad
DynamicJsonObject newTestCaseRef = new DynamicJsonObject();
DynamicJsonObject newTCField = new DynamicJsonObject();
newTCField["_ref"] = TestCaseRef;

RestAPI.Post(Endpoint, newTestCaseRef);

This does not work with current version of 3.0.1. It comes back as unauthorized due to I believe the missing key. The DoPost method is not public, so we cannot use it directly. This seemed like the most logical change