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

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