When initializing the API client, the User-Agent was added to any custom
HTTPHeaders that were configured. However, because the map was not properly
dereferenced, the original map was modified, causing the User-Agent to also
be saved to config.json after `docker login` and `docker logout`:

Before this change;

    $ cat ~/.docker/config.json
    cat: can't open '/root/.docker/config.json': No such file or directory

    $ docker login -u myusername
    Password:
    ...
    Login Succeeded

    $ cat ~/.docker/config.json
    {
        "auths": {
            "https://index.docker.io/v1/": {
                "auth": "<base64 auth>"
            }
        },
        "HttpHeaders": {
            "User-Agent": "Docker-Client/19.03.12 (linux)"
        }
    }

    $ docker logout
    {
        "auths": {},
        "HttpHeaders": {
            "User-Agent": "Docker-Client/19.03.12 (linux)"
        }
    }

After this change:

    $ cat ~/.docker/config.json
    cat: can't open '/root/.docker/config.json': No such file or directory

    $ docker login -u myusername
    Password:
    ...
    Login Succeeded

    $ cat ~/.docker/config.json
    {
        "auths": {
            "https://index.docker.io/v1/": {
                "auth": "<base64 auth>"
            }
        }
    }

    $ docker logout
    Removing login credentials for https://index.docker.io/v1/

    $ cat ~/.docker/config.json
    {
        "auths": {}
    }

Signed-off-by: Sebastiaan van Stijn <[email protected]>