can only safely stream natively on ActionDispatch <= 3.2

since 4.0 Response#stream is added and body is wrapped in a buffer 
(which does more than body.each)

resolves #210 (broken Rails streaming for Rails 4.x)

Author: kares

src/main/ruby/jruby/rack/response.rb

@@ -161,10 +161,11 @@ def write_body(response)
             body = @body.to_inputstream # so that we close the stream
             body = Channels.newChannel(body) # closing the channel closes the stream
             transfer_channel body, response.getOutputStream
-          elsif @body.respond_to?(:body_parts) && @body.body_parts.respond_to?(:to_channel) &&
-              ! object_polluted_with_anyio?(@body.body_parts, :to_channel)
+          elsif @body.respond_to?(:body_parts) && ! @body.respond_to?(:stream) &&
+              ( body_parts = @body.body_parts ).respond_to?(:to_channel) &&
+              ! object_polluted_with_anyio?(body_parts, :to_channel)
             # ActionDispatch::Response "raw" body access in case it's a File
-            body = @body.body_parts.to_channel # so that we close the channel
+            body = body_parts.to_channel # so that we close the channel
             transfer_channel body, response.getOutputStream
           else
             if dechunk?