fix assignment external_tool_tag resetting for rails 3

Change-Id: I0c2c1311caea731dde7822b97876cefdcc45d518
Reviewed-on: https://gerrit.instructure.com/31256
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>