bug#23620: 25.0.94; url-retrieve fails on localhost

[Artur Malabarba]

Assuming you have a webserver serving on localhost:5100, run the
following snippet:

(url-retrieve "http://localhost:5100/";
              (lambda (&rest r)
                (message "%S" r)))

It fails with the following error:
    (:error (error connection-failed "failed with code 111\n"
                   :host "localhost"
                   :service 5100))

Note that if I switch to a terminal and try curl:
    curl "http://localhost:5100/";
it works fine.

Using `url-retrieve' on remote urls also works fine.