Updated the Docker CLI to specify a value for the "Host" header.

At the moment HTTP requests made by the Docker CLI don't specify a value for the *Host* header:

```
$ netcat -l -p 1025
# At this point I executed "docker -H 127.0.0.1:1025 version" in another
# pane.
GET /v1.4/version HTTP/1.1
Host:
User-Agent: Docker-Client/0.5.1
$
```

Some proxies won't have it:

```
# Docker Error
$ docker -H 127.0.0.1:1025 version
2013/08/07 05:25:30 persistent connection closed
$

# nginx Error
2013/08/07 05:25:30 [info] 1241#0: *7 client sent invalid host header while reading client request headers, client: 127.0.0.1, server: , request: "GET /v1.4/version HTTP/1.1", host: ""
```