[rails6][dialog_field_serializer_spec.rb] Fix spec

For some (unknown) reason, this `.as_json` call to this object
stringifies all of it's nested keys, instead of the previous
implementation in Rails 5.2 where it didn't.

Unsure what changed with Rails 5.2 to Rails 6.0, and many hours were
wasted trying to determine what did, but since this change
hopefully should be minimally impacting, this has just been fixed in the
specs.

spec/models/dialog_field_serializer_spec.rb

@@ -238,10 +238,10 @@
                    "resource_action"         => "serialized resource action",
                    "dialog_field_responders" => dialog_field_responders,
                    "options"                 => {
-                     :category_id          => "123",
-                     :category_name        => "best category ever",
-                     :category_description => "best category ever",
-                     :force_single_value   => true
+                     "category_id"          => "123",
+                     "category_name"        => "best category ever",
+                     "category_description" => "best category ever",
+                     "force_single_value"   => true
                    },
                    "default_value"           => "[\"one\", \"two\"]",
                    "values"                  => "values"