class Async::HTTP::Headers

def merge(hash, key, value)
	if policy = MERGE_POLICY[key]
		if current_value = hash[key]
			current_value << value
		else
			hash[key] = policy.new(value)
		end
	else
		raise ArgumentError, "Header #{key} can only be set once!" if hash.include?(key)
		
		# We can't merge these, we only expose the last one set.
		hash[key] = value
	end
end