handle rails 4 flash hashes in session for rails 3

closes #CNVS-20688

Change-Id: Icb9963f78bddb08e014ac480e9eefd030704aea8
Reviewed-on: https://gerrit.instructure.com/67729
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

app/middleware/rails3_flash_shim.rb

@@ -0,0 +1,16 @@
+# remove after switch to Rails 4
+class Rails3FlashShim
+  def initialize(app)
+    @app = app
+  end
+
+  def call(env)
+    if (session = env['rack.session']) && (flash = session['flash'])
+      if flash.is_a?(Hash)
+        session['flash'] = nil
+      end
+    end
+
+    @app.call(env)
+  end
+end

config/application.rb

@@ -118,6 +118,9 @@ module CanvasRails
     initializer("extend_middleware_stack", after: "load_config_initializers") do |app|
       app.config.middleware.insert_before(config.session_store, 'LoadAccount')
       app.config.middleware.insert_before(config.session_store, 'SessionsTimeout')
+      if CANVAS_RAILS3
+        app.config.middleware.insert_before('ActionDispatch::Flash', 'Rails3FlashShim')
+      end
       app.config.middleware.swap('ActionDispatch::RequestId', 'RequestContextGenerator')
       app.config.middleware.insert_after(config.session_store, 'RequestContextSession')
       app.config.middleware.insert_before('ActionDispatch::ParamsParser', 'Canvas::RequestThrottle')