Clear UNIX connections pool to avoid ResourceWarnings

This happens a lot of the time when using the Docker client in any tests:

/home/alexey/.virtualenvs/forklift/lib/python3.4/site-packages/docker/client.py:623: ResourceWarning: unclosed <socket.socket fd=4, family=AddressFamily.AF_UNIX, type=2049, proto=0, raddr=/var/run/docker.sock>

The patch properly cleans up the UNIX socket HTTP connections.