Add missing elements to Excerpt which delegate to its Document

lib/jekyll/drops/drop.rb

@@ -102,8 +102,8 @@ module Jekyll
       #
       # Returns true if the given key is present
       def key?(key)
-        if self.class.mutable && @mutations.key?(key)
-          true
+        if self.class.mutable
+          @mutations.key?(key)
         else
           respond_to?(key) || fallback_data.key?(key)
         end

lib/jekyll/excerpt.rb

@@ -7,7 +7,8 @@ module Jekyll
     attr_writer   :output
 
     def_delegators :@doc, :site, :name, :ext, :relative_path, :extname,
-                          :render_with_liquid?, :collection, :related_posts, :url
+                          :render_with_liquid?, :collection, :related_posts,
+                          :url, :next_doc, :previous_doc
 
     # Initialize this Excerpt instance.
     #