/**
     * Base64 encode user info for HTTP Basic Authentication.
     *
     * Try to use {@literal java.util.Base64} encoder which is available starting with Java 8.
     * Fallback to {@literal javax.xml.bind.DatatypeConverter} from JAXB which is available starting with Java 6 but is not anymore in Java 9.
     * Fortunately, both of these two Base64 encoders implement the right Base64 flavor, the one that does not split the output in multiple lines.