public String getFirst(String key) {
        return h.getFirst(key);
    }
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.
application/src/main/java/com/yahoo/application/container/handler/Headers.java