New: support for flag withCredentials in CPURLConnection.j

CPURLConnection.j: created class variable '_withCredentials' with
accessors to allow user to set the flag accordingly. Overloaded static
methods sendSynchronousRequest and connectionWithRequest to allow users
to provide optional 'withCredentials' argument to set the flag while
creating object.

In instance method 'start' (line 220) set '_withCredentials' flag.

Refs #2138