fix multipart header generation for rails 3 spec compatibility

Change-Id: Idec0d7c04437284703e71130f68e921a5e229785
Reviewed-on: https://gerrit.instructure.com/30799
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>