reset default hash value in sets after yaml deserialization

Change-Id: Iea5625cd6a98082af65a045b63045c36af874c15
Reviewed-on: https://gerrit.instructure.com/201413
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins

lib/canvas_yaml.rb

@@ -242,8 +242,13 @@ SafeYAML::Transform.singleton_class.prepend(ScalarTransformFix)
 
 module YAMLSingletonFix
   def revive(klass, node)
-    return klass.instance if klass < Singleton
-    super
+    if klass < Singleton
+      klass.instance
+    elsif klass == Set
+      super.tap{|s| s.instance_variable_get(:@hash).default = false}
+    else
+      super
+    end
   end
 end
 Psych::Visitors::ToRuby.prepend(YAMLSingletonFix)

spec/lib/safe_yaml_spec.rb

@@ -205,4 +205,8 @@ YAML
   it "dumps and loads singletons" do
     expect(YAML.load(YAML.dump(Mime::NullType.instance))).to eq Mime::NullType.instance
   end
+
+  it "restores default value on sets on load" do
+    expect(YAML.unsafe_load(YAML.dump(Set.new)).include?("test")).to eq false
+  end
 end