Click testing on rails 2.3.10 site works under mod_passenger, selenium tests fail when running under mongrel. Why?

So I'm working on 开发者_开发知识库a large rails app that has a bunch of selenium tests. Running a local dev instance of the site (apache w/ mod_passenger) works fine, but when I run the selenium test suite I get a bunch of failures, particularly on re-directs. The HTTP response header comes back poorly formatted. Among other problems the Set-Cookie header key seems to have been deleted and only the value appears.

PS: I have already solved this problem, just posting the question / answer for the community to see.

Discussion about the answer can be found here:

https://rails.lighthouseapp.com/projects/8994/tickets/4690-mongrel-doesnt-work-with-rails-238

Code for a fix can be found here:

http://gist.github.com/471663