Added method for setting withCredentials. Can be set to either true or f...

[mwq27]

...alse. Assertion added to the test-request-methods.js tests. Closes #71

[kumavis]

I think this is incorrect,
xhr.withCredentials is a boolean that you set, not a function

[kumavis]

Furthermore, it should be passed on to the options for doRequest b/c if undefined, defaults to xhr.withCredentials = true which is not the default behaviour of the browser

node-XMLHttpRequest/lib/XMLHttpRequest.js

Line 447 in f1ced44

request = doRequest(options, responseHandler).on("error", errorHandler);

[driverdan]

Fixed in #96