Fix arguments to 404 response (#63)

The second argument is the headers field, and it's actually a hash type. Sending an empty array breaks parsing further down the line when trying to look up hash values by key.

lib/async/websocket/adapters/rails.rb

@@ -13,7 +13,7 @@ def self.open(request, **options, &block)
 					if response = Rack.open(request.env, **options, &block)
 						::Rack::Response[*response]
 					else
-						::ActionDispatch::Response.new(404, [], [])
+						::ActionDispatch::Response.new(404)
 					end
 				end
 			end