add constants for HTTP-Redirect binding
add constants for HTTP-Redirect binding

Change-Id: Iac178148d97df64e638675e1bca3ce25632ab1d8
Reviewed-on: https://gerrit.instructure.com/49408
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

instructure/ruby-saml2,instructure/ruby-saml2