request.content.iter_chunks() stalls when content is empty.

## Long story short
The title says it all.

Shouldn't [EmptyStreamReader.readchunk](https://github.com/aio-libs/aiohttp/blob/master/aiohttp/streams.py#L470) on its face `return (b'', True)` and not `return (b'', False)` ?

Without it, the special EOS chunk is never sent...