Don't replace empty BODY with empty dict on resp.json() call

[asvetlov]

See #3362 (comment) note.

The change breaks backward compatibility but now the great chance to do it in aiohttp 4.0 :)

[aio-libs-bot]

GitMate.io thinks possibly related issues are #918 (Empty CancelledError while parsing request body (question)), #2143 (FileResponse sends empty chunked body when returning 304), #758 (Empty body handled as error (204 No Content)), #815 (r.json() ), and #1726 (Add API to easily send JSON as request body).

[socketpair]

Instead of empty dict it respond with None.

[socketpair]

Well, just curious... backward compatibility with what ?