fix for rails3 refactoring of TestUploadedFile

in rails 3, ActionController::TestUploadedFile has
been moved to Rack::Test::UploadedFile. This commit
simply updates uses of TestUploadedFile to work
with this new structure

Change-Id: Ib31159c635f033a13908608dffeea88c8f719086
Reviewed-on: https://gerrit.instructure.com/28234
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>