@Override
    public List<String> put(String key, List<String> value) {
        return content.put(key, Collections.synchronizedList(new ArrayList<>(value)));
    }
<p>Convenience method to call {@link #put(String, List)} with a singleton list that contains the specified
value.</p>

@param key   The key of the entry to put.
@param value The value to put.
@return The previous value associated with <code>key</code>, or <code>null</code> if there was no mapping for
        <code>key</code>.
jdisc_core/src/main/java/com/yahoo/jdisc/HeaderFields.java