Fix aiohttp warning due to parsing empty body as JSON.
Fix aiohttp warning due to parsing empty body as JSON.

 * containers.kill API returns HTTP 204 "No Content" when success.

paultag/aiodocker,barrachri/aiodocker,gaopeiliang/aiodocker,barrachri/aiodocker,gaopeiliang/aiodocker,barrachri/aiodocker,gaopeiliang/aiodocker