Fixed: CFHTTPRequest now accepts the withCredentials property

From the original PR:

Currently, CPURLConnection has no way to access the withCredential
property of an XMLHTTPRequest, meaning we can't utilize cross-domain
calls using CORS in our applications.

This commit adds the withCredentials property to CFHTTPRequest.

Refs #1296
Fixed: CPURLConnection now accepts withCredentials

From the original PR:

CPURLConnection was modified to reduce the withCredentials flag to a file-scope member. This has been done to facilitate use of withCredentials when a developer cannot modify the CPURLConnection manually (e.g. https://github.com/wireload/Ratatosk/blob/master/WLRemoteLink.j, line 466).

Refs #1296, #2138