Potentiel data leak in "Invalid form authenticity token" error screen (#16511).

git-svn-id: http://svn.redmine.org/redmine/trunk@13041 e93f8b46-1217-0410-a6f0-8f06a7374b81
Merge pull request #1084 from marutosi/finn-Invalid-form-authenticity-token

[security] Potentiel data leak in "Invalid form authenticity token" error screen