Call overridden version of `to_liquid` and remove duplicate code

Make sure to call "overidden" method which keeps us nice and compatible
so we don't need to keep up with whatever Jekyll usually does, and make
sure to include it in our overriden method.

I have had it with these monkey-patching methods in this Monday-to-
Friday language!

Add method `liquid_date_attributes`.

Cleanup the Post and Page overrides to be more similar.

plugins/date.rb

@@ -41,6 +41,15 @@ module Octopress
       end
       date_formatted
     end
+    
+    # Returns the date-specific liquid attributes
+    def liquid_date_attributes
+      date_format = self.site.config['date_format']
+      data = {}
+      data['date_formatted']    = format_date(self.data['date'], date_format)    if self.data.has_key?('date')
+      data['updated_formatted'] = format_date(self.data['updated'], date_format) if self.data.has_key?('updated')
+      data
+    end
 
   end
 end
@@ -51,48 +60,22 @@ module Jekyll
   class Post
     include Octopress::Date
 
-    # Convert this post into a Hash for use in Liquid templates.
-    #
-    # Returns <Hash>
+    # Convert this Convertible's data to a Hash suitable for use by Liquid.
+    # Overrides the default return data and adds any date-specific liquid attributes
+    alias :super_to_liquid :to_liquid
     def to_liquid
-      date_format = self.site.config['date_format']
-      self.data.deep_merge({
-        "title"             => self.data['title'] || self.slug.split('-').select {|w| w.capitalize! || w }.join(' '),
-        "url"               => self.url,
-        "date"              => self.date,
-        # Monkey patch
-        "date_formatted"    => format_date(self.date, date_format),
-        "updated_formatted" => self.data.has_key?('updated') ? format_date(self.data['updated'], date_format) : nil,
-        "id"                => self.id,
-        "categories"        => self.categories,
-        "next"              => self.next,
-        "previous"          => self.previous,
-        "tags"              => self.tags,
-        "content"           => self.content })
+      super_to_liquid.deep_merge(liquid_date_attributes)
     end
   end
 
   class Page
     include Octopress::Date
 
-    # Initialize a new Page.
-    #
-    # site - The Site object.
-    # base - The String path to the source.
-    # dir  - The String path between the source and the file.
-    # name - The String filename of the file.
-    def initialize(site, base, dir, name)
-      @site = site
-      @base = base
-      @dir  = dir
-      @name = name
-
-      self.process(name)
-      self.read_yaml(File.join(base, dir), name)
-      # Monkey patch
-      date_format = self.site.config['date_format']
-      self.data['date_formatted']    = format_date(self.data['date'], date_format) if self.data.has_key?('date')
-      self.data['updated_formatted'] = format_date(self.data['updated'], date_format) if self.data.has_key?('updated')
+    # Convert this Convertible's data to a Hash suitable for use by Liquid.
+    # Overrides the default return data and adds any date-specific liquid attributes
+    alias :super_to_liquid :to_liquid
+    def to_liquid
+      super_to_liquid.deep_merge(liquid_date_attributes)
     end
   end
 end