pages' permalink' extnames must be respected

This reverts a bit of the work @willnorris had made to support
extensionless permalinks. Using the ‘permalink’ front matter will no
longer work as it must allow non-html extensions to be written.

test/source/+/foo.md

@@ -1,7 +1,7 @@
 ---
 layout: default
 title : Page inside +
-permalink: /+/plus+in+url
+permalink: /+/plus+in+url.html
 ---
 Line 1
 {{ page.title }}

test/source/deal.with.dots.html

@@ -1,6 +1,5 @@
 ---
 title: Deal with dots
-permalink: /deal.with.dots
 ---
 
 Let's test if jekyll deals properly with dots.

test/test_page.rb

@@ -69,7 +69,6 @@ class TestPage < JekyllUnitTest
         @dest_file = dest_dir("deal.with.dots.html")
 
         assert_equal "deal.with.dots", @page.basename
-        assert_equal "/deal.with.dots", @page.url
         assert_equal @dest_file, @page.destination(dest_dir)
       end