fix pseudonym_sessions_controller_spec for rails 3

Change-Id: I54e9874ae78d3f47d4392aefecb8f0668232ef1a
Reviewed-on: https://gerrit.instructure.com/30270
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>