Use root schema for custom keywords

`custom_keywords` isn't passed to nested schemas (similar to formats,
hooks, and ref resolution) so they need to be read from the root schema
instead.

Noticed this while looking into: #166

lib/json_schemer/schema.rb

@@ -148,8 +148,8 @@ def validate_instance(instance, instance_location, keyword_location, context)
           adjacent_results[keyword_instance.class] = keyword_result
         end
 
-        if custom_keywords.any?
-          custom_keywords.each do |custom_keyword, callable|
+        if root.custom_keywords.any?
+          root.custom_keywords.each do |custom_keyword, callable|
             if value.key?(custom_keyword)
               [*callable.call(instance, value, instance_location)].each do |custom_keyword_result|
                 custom_keyword_valid = custom_keyword_result == true

test/json_schemer_test.rb

@@ -209,6 +209,25 @@ def test_it_validates_correctly_custom_keywords
     refute(schema.valid?(3))
   end
 
+  def test_it_validates_custom_keywords_in_nested_schemas
+    schemer = JSONSchemer.schema(
+      {
+        'properties' => {
+          'x' => {
+            'yah' => true
+          }
+        }
+      },
+      keywords: {
+        'yah' => proc do |instance, _schema, _instance_location|
+          instance == 'valid'
+        end
+      }
+    )
+    assert(schemer.valid?({ 'x' => 'valid' }))
+    refute(schemer.valid?({ 'x' => 'invalid' }))
+  end
+
   def test_it_handles_multiple_of_floats
     assert(JSONSchemer.schema({ 'multipleOf' => 0.01 }).valid?(8.61))
     refute(JSONSchemer.schema({ 'multipleOf' => 0.01 }).valid?(8.666))