RackResponse 304 output not compatible with XMLHTTPRequest spec

This is the relevant section:

The XML response entity body is either a Document representing the response entity body or null. The XML response entity body is the return value of the following algorithm:

1. If the response entity body is "null" terminate these steps and return null.

2. If a Content-Type is present and it does not contain a MIME type (ignoring any parameters) that is text/xml, application/xml or ends in +xml terminate these steps and return null. (Do not terminate these steps if there is no Content-Type header at all.)

The problem is that rack_process.rb deletes the content type and sets the output to [] which returns an empty response body. Because it removes the content-type header, XMLHTTPRequest will interpret the response as text/xml. An empty string is not valid XML and will produce a "no element found" error in Firefox. It seems that the out/to_a method should set the content type to something like text/plain or return a response body of "null" for 304 status responses.

We use AJAX calls to return JSON data to a grid and would like to be able to use etags to keep the load/traffic down on the server.