append(name, value) {
		name = `${name}`;
		value = `${value}`;
		validateName$5(name);
		validateValue$5(value);
		const key = find$5(this[MAP$5], name);
		if (key !== undefined) {
			this[MAP$5][key].push(value);
		} else {
			this[MAP$5][name] = [value];
		}
	}
Append a value onto existing header

@param   String  name   Header name
@param   String  value  Header value
@return  Void