Completely reset the session, including cookies.

I found that if the session contains a cookie, that cookie is not removed unless -resetWithCompletionHandler: is called to completely clear it.

I made the change internally to my app, but it seems like this should be handled by AFNetworking itself. I could make the complete reset a parameter, but I don't think that breaking the API is the best thing to do.

It looks like -resetWithCompletionHandler: is the only API documented as clearing the cookies.

Thanks for making AFNetworking!