care for 'Content-Length'  on taskDidSendBodyData block

continue on the fix for the progress to work on Multipart requests that this PR [#1617](https://github.com/AFNetworking/AFNetworking/pull/1617) solved, it will be nice if the logic can move up, to also benefit the 'taskDidSendBodyData' block on the manager, so that both the delegate and the block(if set),  are in-sync on the progress size.