Allow exclusive refs into sibling definitions

I don't think this is really correct according to the
[specification][0], but it matches the previous behavior and seems
useful. Drafts after 7 don't have a problem because they allow all
keywords as `$ref` siblings.

Related:

- #44 (comment)
- json-schema-org/JSON-Schema-Test-Suite#458

[0]: https://datatracker.ietf.org/doc/html/draft-handrews-json-schema-01#section-8.3

lib/json_schemer/schema.rb

@@ -325,6 +325,10 @@ def parse
 
         if value.key?('$ref') && keywords.fetch('$ref').exclusive?
           @parsed['$ref'] = keywords.fetch('$ref').new(value.fetch('$ref'), self, '$ref')
+          defs_keyword = meta_schema.defs_keyword
+          if value.key?(defs_keyword) && keywords.key?(defs_keyword)
+            @parsed[defs_keyword] = keywords.fetch(defs_keyword).new(value.fetch(defs_keyword), self, defs_keyword)
+          end
         else
           keyword_order = meta_schema.keyword_order
           last = keywords.size

test/ref_test.rb

@@ -364,4 +364,19 @@ def test_it_handles_relative_base_uri_json_pointer_ref
     assert(schema.valid?({ 'bar' => 1 }))
     refute(schema.valid?({ 'bar' => '1' }))
   end
+
+  def test_exclusive_ref_supports_definitions
+    schema = JSONSchemer.schema({
+      '$schema' => 'http://json-schema.org/draft-07/schema#',
+      '$ref' => '#yah',
+      'definitions' => {
+        'yah' => {
+          '$id' => '#yah',
+          'type' => 'integer'
+        }
+      }
+    })
+    assert(schema.valid?(1))
+    refute(schema.valid?('1'))
+  end
 end