Monitoring systems and load balancers are usually configured to use HEAD
requests for health monitoring. The /_ping endpoint currently does not
support this type of request, which means that those systems have fallback
to GET requests.

This patch adds support for HEAD requests on the /_ping endpoint.

Although optional, this patch also returns `Content-Type` and `Content-Length`
headers in case of a HEAD request; Refering to RFC 7231, section 4.3.2:

    The HEAD method is identical to GET except that the server MUST NOT
    send a message body in the response (i.e., the response terminates at
    the end of the header section).  The server SHOULD send the same
    header fields in response to a HEAD request as it would have sent if
    the request had been a GET, except that the payload header fields
    (Section 3.3) MAY be omitted.  This method can be used for obtaining
    metadata about the selected representation without transferring the
    representation data and is often used for testing hypertext links for
    validity, accessibility, and recent modification.

    A payload within a HEAD request message has no defined semantics;
    sending a payload body on a HEAD request might cause some existing
    implementations to reject the request.

    The response to a HEAD request is cacheable; a cache MAY use it to
    satisfy subsequent HEAD requests unless otherwise indicated by the
    Cache-Control header field (Section 5.2 of [RFC7234]).  A HEAD
    response might also have an effect on previously cached responses to
    GET; see Section 4.3.5 of [RFC7234].

With this patch applied, either `GET` or `HEAD` requests work; the only
difference is that the body is empty in case of a `HEAD` request;

    curl -i --unix-socket /var/run/docker.sock http://localhost/_ping
    HTTP/1.1 200 OK
    Api-Version: 1.40
    Cache-Control: no-cache, no-store, must-revalidate
    Docker-Experimental: false
    Ostype: linux
    Pragma: no-cache
    Server: Docker/dev (linux)
    Date: Mon, 14 Jan 2019 12:35:16 GMT
    Content-Length: 2
    Content-Type: text/plain; charset=utf-8

    OK

    curl --head -i --unix-socket /var/run/docker.sock http://localhost/_ping
    HTTP/1.1 200 OK
    Api-Version: 1.40
    Cache-Control: no-cache, no-store, must-revalidate
    Content-Length: 0
    Content-Type: text/plain; charset=utf-8
    Docker-Experimental: false
    Ostype: linux
    Pragma: no-cache
    Server: Docker/dev (linux)
    Date: Mon, 14 Jan 2019 12:34:15 GMT

The client is also updated to use `HEAD` by default, but fallback to `GET`
if the daemon does not support this method.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>