public String getFirst(String key) {
        List<String> lst = get(key);
        if (lst == null || lst.isEmpty()) {
            return null;
        }
        return lst.get(0);
    }
Convenience method for retrieving the first value of a named header field. If the header is not set, or if the
value list is empty, this method returns null.

@param key the key whose first value to return
@return the first value of the named header, or null
jdisc_core/src/main/java/com/yahoo/jdisc/HeaderFields.java