resolveOne (url) {
        // url must be either string or an instance of url.
        if (!_.isString(url) && !Url.isUrl(url)) {
            return;
        }

        // find a certificate that can be applied to the url
        return this.find(function (certificate) {
            return certificate.canApplyTo(url);
        });
    }
Matches the given url against the member certificates' allowed matches
and returns the certificate that can be used for the url.

@param {String} url The url to find the certificate for
@returns {Certificate.definition=} The matched certificate