POST requests broken on Passenger

Given a brand new rails project:

rails viewtest
cd viewtest
rake rails:freeze:edge
rake rails:update:application_controller
script/generate controller test

app/view/test/index.html.erb:

<% form_tag do %>
  <%= submit_tag 'Go' %>
<% end %>

Minimal passenger configuration:

<VirtualHost *:80>
  ServerName viewtest.local
  DocumentRoot "/Users/sam/viewtest/public"

  <Directory "/Users/sam/viewtest/public">
    Order allow,deny
    Allow from all
  </Directory>
</VirtualHost>

GET /test works. Any POST to /test (submitting the form) breaks. development.log with backtrace is attached. No issues running through script/server however.

This seems to have been caused by this commit.