Fix arg

Author: daikimiura

lib/openai/stream.rb

@@ -17,7 +17,7 @@ def initialize(user_proc:, parser: EventStreamParser::Parser.new)
         end
     end
 
-    def call(chunk, _bytes, env)
+    def call(chunk, _bytes, env = nil)
       handle_http_error(chunk: chunk, env: env) if env && env.status != 200
 
       parser.feed(chunk) do |event, data|