public void add(String key, List<String> values) {
        List<String> lst = content.get(key);
        if (lst != null) {
            lst.addAll(values);
        } else {
            put(key, values);
        }
    }
Adds the given values to the entry of the specified key. If no entry exists for the given key, a new one is
created containing only the given values.

@param key    the key with which the specified value is to be associated.
@param values the values to be added to the list associated with the specified key.
jdisc_core/src/main/java/com/yahoo/jdisc/HeaderFields.java