allow GET request for SAML logout

fixes: CNVS-14491

Allows GET requests for logout only when a SAML request is sent.
Still only allows DELETE methods for all other authentication types.

Test Plan:

Initiate a SAML logout, Should work.
Any other authentications methods should only logout with DELETE methods.

Reference test plan for g/32723.

Change-Id: Ia9153927e1ec17189d0fa00084837d451a1281a3
Reviewed-on: https://gerrit.instructure.com/38837
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>