Ignore $id when $ref is present

I guess `$ref` isn't supposed to take sibling `$id` values into account.
This switches to modifying the current `parent_uri` after `$ref` is
processed. I still needed to update the root `parent_uri` to support
`test_it_handles_nested_refs`, when the schema has an `$id` based on
where it was fetched.

Addresses failures from: json-schema-org/JSON-Schema-Test-Suite#493

Author: davishmcclurg

lib/json_schemer/schema/base.rb

@@ -102,13 +102,14 @@ def validate_instance(instance, &block)
         ref = schema['$ref']
         id = schema[id_keyword]
 
-        instance.parent_uri = join_uri(instance.parent_uri, id)
-
         if ref
+          instance.parent_uri ||= URI.parse(root[id_keyword]) if root.key?(id_keyword)
           validate_ref(instance, ref, &block)
           return
         end
 
+        instance.parent_uri = join_uri(instance.parent_uri, id)
+
         if format? && custom_format?(format)
           validate_custom_format(instance, formats.fetch(format), &block)
         end