use Kernel#Array instead of explicit Array check (#6525)

Merge pull request 6525

lib/jekyll/configuration.rb

@@ -164,8 +164,7 @@ module Jekyll
         config_files = Jekyll.sanitized_path(source(override), "_config.#{default}")
         @default_config_file = true
       end
-      config_files = [config_files] unless config_files.is_a? Array
-      config_files
+      Array(config_files)
     end
 
     # Public: Read configuration and return merged Hash