ConnectionResetError when close browser, but not all static files was fetch

#1773 - The problem still remains in ver. 2.1.0.dev0:

```
RuntimeError: File descriptor 12 is used by transport <_SelectorSocketTransport fd=12 read=polling write=>
Exception in callback SendfilePayloadWriter._sendfile_cb(, 12, 9, 30240, 1018100, <_UnixSelecto...se debug=True>, True)
handle: , 12, 9, 30240, 1018100, <_UnixSelecto...se debug=True>, True) created at /usr/local/lib/python3.6/asyncio/selector_events.py:298>
source_traceback: Object created at (most recent call last):
File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/alfa_promo/__main__.py", line 6, in
run_app(alfa_promo.application, host='0.0.0.0', port=5897)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web.py", line 413, in run_app
loop.run_forever()
File "/usr/local/lib/python3.6/asyncio/base_events.py", line 421, in run_forever
self._run_once()
File "/usr/local/lib/python3.6/asyncio/base_events.py", line 1417, in _run_once
handle._run()
File "/usr/local/lib/python3.6/asyncio/events.py", line 126, in _run
self._callback(*self._args)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web_protocol.py", line 432, in start
yield from resp.prepare(request)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web_fileresponse.py", line 238, in prepare
return (yield from self._sendfile(request, fobj, count))
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web_fileresponse.py", line 126, in _sendfile_system
yield from writer.sendfile(fobj, count)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web_fileresponse.py", line 75, in sendfile
self._sendfile_cb(fut, out_fd, in_fd, offset, count, loop, False)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web_fileresponse.py", line 53, in _sendfile_cb
offset + n, count - n, loop, True)
File "/usr/local/lib/python3.6/asyncio/selector_events.py", line 347, in add_writer
return self._add_writer(fd, callback, *args)
File "/usr/local/lib/python3.6/asyncio/selector_events.py", line 298, in _add_writer
handle = events.Handle(callback, args, self)
Traceback (most recent call last):
File "/usr/local/lib/python3.6/asyncio/events.py", line 126, in _run
self._callback(*self._args)
File "/root/.virtualenvs/36/root/lib/python3.6/site-packages/aiohttp/web_fileresponse.py", line 37, in _sendfile_cb
loop.remove_writer(out_fd)
File "/usr/local/lib/python3.6/asyncio/selector_events.py", line 351, in remove_writer
self._ensure_fd_no_transport(fd)
File "/usr/local/lib/python3.6/asyncio/selector_events.py", line 258, in _ensure_fd_no_transport
fd, transport))
```