Unify atom links on index page and atom itself.

The atom link on the html index (without tag) is "url/posts.atom" while on the atom feed the self link was "url/.atom".

This commit copies the url statement from index to atom feed.

app/views/posts/index.atom.builder

@@ -1,5 +1,11 @@
+url = if @tag.nil?
+  formatted_posts_path(:format => 'atom', :only_path => false)
+else
+  posts_path(:tag => @tag, :format => 'atom', :only_path => false)
+end
+
 atom_feed(
-  :url         => posts_path(:tag => @tag, :format => 'atom', :only_path => false),
+  :url         => url,
   :root_url    => posts_path(:tag => @tag, :only_path => false),
   :schema_date => '2008'
 ) do |feed|