Fix an issue with the reachability manager not monitoring

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).