separated up the attrs required by convertible and made tags and categories always non-nil as empty front matter entries could cause them to be nil

lib/jekyll/convertible.rb

@@ -3,6 +3,10 @@
 #
 # Requires
 #   self.site -> Jekyll::Site
+#   self.content=
+#   self.data=
+#   self.ext=
+#   self.output=
 module Jekyll
   module Convertible
     # Return the contents as a string

lib/jekyll/post.rb

@@ -18,12 +18,9 @@ module Jekyll
       name =~ MATCHER
     end
 
-    attr_accessor :site, :date, :slug, :ext, :published, :data, :content, :output, :tags
-    attr_writer :categories
-
-    def categories
-      @categories ||= []
-    end
+    attr_accessor :site
+    attr_accessor :data, :content, :output, :ext
+    attr_accessor :date, :slug, :published, :tags, :categories
 
     # Initialize this Post instance.
     #   +site+ is the Site
@@ -51,9 +48,8 @@ module Jekyll
         self.tags = [self.data["tag"]]
       elsif self.data.has_key?("tags")
         self.tags = self.data['tags']
-      else
-        self.tags = []
       end
+      self.tags ||= []
 
       if self.categories.empty?
         if self.data.has_key?('category')
@@ -68,6 +64,7 @@ module Jekyll
           end
         end
       end
+      self.categories ||= []
     end
 
     # Spaceship is based on Post#date, slug

test/test_post.rb

@@ -36,6 +36,7 @@ class TestPost < Test::Unit::TestCase
       end
 
       should "keep date, title, and markup type" do
+        @post.categories = []
         @post.process(@fake_file)
 
         assert_equal Time.parse("2008-10-19"), @post.date
@@ -202,6 +203,13 @@ class TestPost < Test::Unit::TestCase
     end
 
     context "initializing posts" do
+
+      should "ensure suitable defaults for attributes" do
+        post = setup_post("2009-06-22-no-yaml.textile")
+        assert_equal [], post.tags
+        assert_equal [], post.categories
+      end
+
       should "publish when published yaml is no specified" do
         post = setup_post("2008-02-02-published.textile")
         assert_equal true, post.published