make serializable_hash always return a hash with indifferent access

flag=none

Previously, the method would only return a hash with indifferent access
when include_root was false. When include_root was true, it would return
a plain-old hash. This change makes the return value type consistent, so
that it always returns a hash with indifferent access.

Test Plan:
- specs pass

Change-Id: I8fbc23523f1476a45a592cd02884d84d7c5a85e3
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/334369
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Omar Soto-Fortuño <omar.soto@instructure.com>
Product-Review: Omar Soto-Fortuño <omar.soto@instructure.com>

config/initializers/active_record.rb

@@ -2119,7 +2119,7 @@ module UserContentSerialization
       end
     end
     if options && options[:include_root]
-      result = { self.class.base_class.model_name.element => result }
+      result = { self.class.base_class.model_name.element => result }.with_indifferent_access
     end
     result
   end

spec/initializers/active_record_spec.rb

@@ -19,6 +19,20 @@
 
 module ActiveRecord
   describe Base do
+    describe ".serializable_hash" do
+      let(:account) { Account.create! }
+
+      it "returns a hash with indifferent access when the root is included" do
+        hash = account.serializable_hash(include_root: true)
+        expect(hash).to be_a ActiveSupport::HashWithIndifferentAccess
+      end
+
+      it "returns a hash with indifferent access when the root is excluded" do
+        hash = account.serializable_hash(include_root: false)
+        expect(hash).to be_a ActiveSupport::HashWithIndifferentAccess
+      end
+    end
+
     describe ".wildcard" do
       it "produces a useful wildcard sql string" do
         sql = Base.wildcard("users.name", "users.short_name", "Sinatra, Frank", delimiter: ",")