@Override
    public String toString() {
        StringBuilder ret = new StringBuilder();
        toConfig(ret, "");
        return ret.toString();
    }
Convert a string value to a quoted value suitable for use in a config string.
<p>
Adds double quotes before and after, and adds backslash-escapes to any double quotes that was contained in the
string.  A null pointer will produce the special unquoted string null that the config library will convert back
to a null pointer.

@param input the String to be escaped
@return an escaped String
messagebus/src/main/java/com/yahoo/messagebus/routing/RoutingSpec.java