use canvas' http library for logout service callback

Raw net::http doesn't support ssl

Test Plan:
 * Test LTI logout service callback to an https endpoint

closes CNVS-16588

Change-Id: I0da743a6cf192b412f7387c37d5278154ba0018e
Reviewed-on: https://gerrit.instructure.com/43602
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>

app/models/lti/logout_service.rb

@@ -49,7 +49,7 @@ module Lti
       def perform
         callbacks.each do |tool_id, callback|
           begin
-            Net::HTTP::get(URI.parse(callback))
+            CanvasHttp.get(URI.parse(callback).to_s)
           rescue => e
             Rails.logger.error("Failed to call logout callback '#{callback}': #{e.inspect}")
           end

spec/apis/lti/logout_service_spec.rb

@@ -117,7 +117,7 @@ describe LtiApiController, type: :request do
       login_as 'parajsa', 'password1'
       token = Lti::LogoutService::Token.create(@tool, @pseudonym)
       Lti::LogoutService.register_logout_callback(token, 'http://logout.notify.example.com/789')
-      Net::HTTP.expects(:get).with(URI('http://logout.notify.example.com/789'))
+      CanvasHttp.expects(:get).with('http://logout.notify.example.com/789')
       delete '/logout'
       run_jobs
     end