== In short ==
I only found how to reproduce the bug with more details. A patch is not provided.
Chromium on 1st request, only loads the page at the given url as expected.
On refresh, it will first issue a GET /favicon.ico request, which is forwarded to the nginx proxy. Then GET /tests/index.html is issued. Because the connection is kept-alive by nginx and because mongrel2 doesn't inspect the traffic (I guess), the GET /tests/index.html will be forwarded to nginx on the same channel, bypassing the mongrel2 routing.
== In long ==