Move dispatch_group_leave to writeSource cancel handler
Move dispatch_group_leave to writeSource cancel handler

This ensures that the dispatch_group is not deallocated until it is empty

brittonmobile/DLSFTPClient,brittonmobile/DLSFTPClient,dleehr/DLSFTPClient,brittonmobile/DLSFTPClient,dleehr/DLSFTPClient,dleehr/DLSFTPClient