Make permalink parsing consistent with pages

Require permalink to end in a slash to write an index file.
Addresses #2920.

lib/jekyll/post.rb

@@ -269,7 +269,7 @@ module Jekyll
     def destination(dest)
       # The url needs to be unescaped in order to preserve the correct filename
       path = Jekyll.sanitized_path(dest, URL.unescape_path(url))
-      path = File.join(path, "index.html") if path[/\.html?$/].nil?
+      path = File.join(path, "index.html") if self.url =~ /\/$/
       path
     end
 

test/test_post.rb

@@ -589,7 +589,7 @@ class TestPost < Test::Unit::TestCase
 
         should "write properly without html extension" do
           post = setup_post("2008-10-18-foo-bar.textile")
-          post.site.permalink_style = ":title"
+          post.site.permalink_style = ":title/"
           do_render(post)
           post.write(dest_dir)