diff --git a/lib/jekyll/drops/collection_drop.rb b/lib/jekyll/drops/collection_drop.rb
index 8dc8c6b8..b8a1bdd7 100644
--- a/lib/jekyll/drops/collection_drop.rb
+++ b/lib/jekyll/drops/collection_drop.rb
@@ -10,7 +10,7 @@ module Jekyll
                             :relative_directory
 
       def to_s
-        @obj.docs.map(&:to_s).join(' ')
+        docs.to_s
       end
 
       def output
diff --git a/lib/jekyll/drops/document_drop.rb b/lib/jekyll/drops/document_drop.rb
index c4170d86..0da6986b 100644
--- a/lib/jekyll/drops/document_drop.rb
+++ b/lib/jekyll/drops/document_drop.rb
@@ -9,10 +9,6 @@ module Jekyll
 
       alias_method :path, :relative_path
 
-      def url
-        @obj.url
-      end
-
       def collection
         @obj.collection.label
       end
@@ -30,9 +26,7 @@ module Jekyll
       end
 
       private
-      def fallback_data
-        @obj.data
-      end
+      def_delegator :@obj, :data, :fallback_data
 
     end
   end
diff --git a/lib/jekyll/drops/site_drop.rb b/lib/jekyll/drops/site_drop.rb
index 82c55230..4c632a0d 100644
--- a/lib/jekyll/drops/site_drop.rb
+++ b/lib/jekyll/drops/site_drop.rb
@@ -6,10 +6,11 @@ module Jekyll
       extend Forwardable
 
       def_delegator  :@obj, :site_data, :data
-      def_delegators :@obj, :time, :pages, :static_files, :documents
+      def_delegators :@obj, :time, :pages, :static_files, :documents,
+                            :tags, :categories
 
       def [](key)
-        if !respond_to?(key) && @obj.collections.key?(key)
+        if @obj.collections.key?(key) && key != "posts"
           @obj.collections[key].docs
         else
           super(key)
@@ -24,14 +25,6 @@ module Jekyll
         @site_html_pages ||= @obj.pages.select { |page| page.html? || page.url.end_with?("/") }
       end
 
-      def categories
-        @site_categories ||= @obj.post_attr_hash('categories')
-      end
-
-      def tags
-        @site_tags ||= @obj.post_attr_hash('tags')
-      end
-
       def collections
         @site_collections ||= @obj.collections.values.map(&:to_liquid)
       end
diff --git a/lib/jekyll/drops/unified_payload_drop.rb b/lib/jekyll/drops/unified_payload_drop.rb
index 65733b20..6db48f66 100644
--- a/lib/jekyll/drops/unified_payload_drop.rb
+++ b/lib/jekyll/drops/unified_payload_drop.rb
@@ -2,7 +2,7 @@
 
 module Jekyll
   module Drops
-    class UnifiedPayloadDrop < Liquid::Drop
+    class UnifiedPayloadDrop < ImmutableDrop
 
       attr_accessor :page, :layout, :content, :paginator
       attr_accessor :highlighter_prefix, :highlighter_suffix
@@ -19,6 +19,11 @@ module Jekyll
         @site_drop ||= SiteDrop.new(@site)
       end
 
+      private
+      def fallback_data
+        @fallback_data ||= {}
+      end
+
     end
   end
 end