I explicitly added version 1.3.1 of WEBrick to my gemfile and this solved the problem.
Strange: According to the message that displays when I run rails s, I was already using WEBrick 1.3.1 (with Rails 3.2.8), but I was still getting the warning messages. I added gem 'webrick', '1.3.1' to my gemfile and the warnings stopped. A different version of 1.3.1?
This did NOT fix it for me, perhaps because I am using RVM. wasn't sure where to make this change with RVM. However putting gem 'webrick', '1.3.1' in my Gemfile and bundle install and restart server worked for me and removed the message.