Warning about already initialized constant PDF

This error message appears when I deploy in stating or production:

/usr/local/sapos/staging/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.13/lib/action_dispatch/http/mime_type.rb:102: warning: already initialized constant PDF
/usr/local/sapos/staging/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.13/lib/action_dispatch/http/mime_type.rb:102: warning: already initialized constant PDF
/usr/local/sapos/staging/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.13/lib/action_dispatch/http/mime_type.rb:102: warning: already initialized constant PDF
/usr/local/sapos/staging/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.13/lib/action_dispatch/http/mime_type.rb:102: warning: already initialized constant PDF

Here is some hint about the problem: http://stackoverflow.com/questions/10944399/warning-already-initialized-constant-pdf