WARN Could not determine content-length of response body. Set content-length of the response or set Response#chunked = true [duplicate]
This is a problem of Webrick. you can use "Thin" instead.
Add this to Gemfile
gem 'thin'
then rails s will use thin instead of Webrick, and the warn will disappear.
Patch mentioned by Amiel Martin works for me! Just find your webrick path (ex., c:\Ruby\lib\ruby\1.9.1\webrick\ on Windows) and change httpresponse.rb file as described in https://bugs.ruby-lang.org/attachments/2300/204_304_keep_alive.patch
And don't forget restart Webrick!