This auth handler adds the Authorization header to the first
HTTP request rather than waiting for a HTTP 401 Unauthorized
response from the server as the default HTTPBasicAuthHandler
does.

This allows working with websites like https://api.github.com which do
not follow the strict interpretation of RFC, but more the dicta in the
end of section 2 of RFC 2617:

    > A client MAY preemptively send the corresponding Authorization
    > header with requests for resources in that space without receipt
    > of another challenge from the server.  Similarly, when a client
    > sends a request to a proxy, it may reuse a userid and password in
    > the Proxy-Authorization header field without receiving another
    > challenge from the proxy server. See section 4 for security
    > considerations associated with Basic authentication.

Patch by Matej Cepl.