Fix an issue with the reachability manager not monitoring in AFHTTPSessionManager

AFURLSessionManager re-declared a private property of it’s superclass and then incorrectly set it after super’s init which resulted in the shared manager already being started and potentially not even used. The newly set reachability manager in AFHTTPSessionManager wasn’t started.

This change allows passing AFURLSessionManager a reachability manager instead of having to redeclare private API (and incorrectly use it).