implement HTTP POST binding
implement HTTP POST binding

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Change-Id: I9d6df2c8d04b5e7e8b79c0c2e4172be1d929d299
Reviewed-on: https://gerrit.instructure.com/133097
Tested-by: Jenkins
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

instructure/ruby-saml2,instructure/ruby-saml2