apply Rails security patch

see https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/DCNTNp_qjFM

Change-Id: I711f1182195d1847408636ab2f9e77a7f0047ac3
Reviewed-on: https://gerrit.instructure.com/16478
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Stanley Stuart <stanley+gerrit@instructure.com>
Reviewed-by: Stanley Stuart <stanley+gerrit@instructure.com>
treesame commit of origin/stable/2012-12-22

Treesame-Commit-Id: 2a1ca6c06065fdb2b048add069a8d2edd64f035f