re-allow POST for saml_consume

fixes CNVS-16409

Change-Id: I4b4d732d4cc4be0a171b53fe4327694b96177edb
Reviewed-on: https://gerrit.instructure.com/43276
Reviewed-by: Paul Hinze <paulh@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>