rails3: fix env key name for path_parameters in middleware

- fixes track_memory_and_cpu_spec failure

Change-Id: Ia8faa8a935df7d2bfbd7dc8a778b151dbcd4abeb
Reviewed-on: https://gerrit.instructure.com/30237
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>