The select_proxy utility in requests errors out when the provided URL
doesn't have a hostname, like is the case when using a UNIX socket.
Since proxies are an irrelevant notion in the case of UNIX sockets
anyway, we simply return the path URL directly.

Signed-off-by: Joffrey F <joffrey@docker.com>

Conflicts:
	docker/client.py
	docker/unixconn/unixconn.py
	tests/integration_test.py

This allows streaming a system to the 'import' command, rather than
having to read the whole thing into memory before sending it.

Previously both the UnixAdapter and the docker.Client objects would
track the 'base URL' of the Docker daemon (socket path in the case of
local Unix-domain socket connections). The Client object would construct
URLs which contained the path to the socket with the path of the Docker
API call appended. The UnixHTTPConnection instance would then remove the
known socket path from the URL.

This relied on all calls going through the HTTPConnection.request()
function, where the URL could be rewritten. In the case of 'chunked'
HTTP POST requests this doesn't happen, so such calls would request
a path still including the socket path and would receive a 404 error.

The client now constructs URLs containing just the path of the desired API
endpoint, and expects the Unix socket transport to know the path to the
Docker daemon's socket.

Next rules were broken
* E265 block comment should start with '# '
* E713 test for membership should be 'not in'

Starting with requests 2.1.0 non-HTTP scheme URLs are not parsed anymore.
The net effect of this is that when using the unix socket in docker-py no
query string params are passed in the URL.  This change makes docker-py
internally use the scheme http+unix to indicate to requests that this is
still a HTTP URL and should be parsed.

Users of docker-py can still specify 'unix:' as the base_url.  The
following forms of base_url are accepted.

  http+unix://var/run/docker.sock
  unix://var/run/docker.sock
  unix:///var/run/docker.sock
  http://hostname
  tcp://hostname

Conflicts:
	docker/client.py
	tests/integration_test.py
	tests/test.py