Improve :failed_attribute value

When this failed_attribute param is parsed, it expects a class name, not an instance name. This will ensure it shows `CustomFormat` instead of `0x00009783abf73>`

Author: bmdelaune

lib/json-schema/attributes/formats/custom.rb

@@ -13,7 +13,7 @@ def validate(current_schema, data, fragments, processor, _validator, options = {
           @validation_proc.call data
         rescue JSON::Schema::CustomFormatError => e
           message = "The property '#{self.class.build_fragment(fragments)}' #{e.message}"
-          self.class.validation_error(processor, message, fragments, current_schema, self, options[:record_errors])
+          self.class.validation_error(processor, message, fragments, current_schema, self.class, options[:record_errors])
         end
       end
     end