DocumentDrop: use def_delegators instead of duplicating methods

lib/jekyll/drops/document_drop.rb

@@ -3,18 +3,10 @@
 module Jekyll
   module Drops
     class DocumentDrop < ImmutableDrop
+      extend Forwardable
 
-      def output
-        @obj.output
-      end
+      def_delegators :@obj, :id, :output, :content, :to_s, :relative_path, :url
 
-      def content
-        @obj.content
-      end
-
-      def relative_path
-        @obj.relative_path
-      end
       alias_method :path, :relative_path
 
       def url
@@ -33,10 +25,6 @@ module Jekyll
         @obj.previous_doc
       end
 
-      def id
-        @obj.id
-      end
-
       def excerpt
         fallback_data['excerpt'].to_s
       end