Rake "already initialized constant WFKV_" warning

Rack 1.3.5 is out now, which has fixed this warning.

I started having the same problem this evening. It seems to be related to Rack 1.3.4. I fixed it by adding this to my Gemfile:

gem 'rack', '1.3.3'

Then running:

bundle update rack

Incidentally, I tried Bozhidar's suggestion before this, but to no avail.