Always close implicit connector, even for keepalive responses.

If network resource returns keepalive response an *implicit* connection is not closed now.
That's leads to resource leaks and eventually to `OSError: [Errno 24] Too many open files`.
I guess to always close *implicit* connection and process keepalives only if *explicit*  connector has been passed to `aiohttp.request`.

See http://stackoverflow.com/questions/24298095/connections-arent-closing-with-python3-asyncio-concurrent-http-get-requests/24312441?noredirect=1 as initial report for the issue