Consolidate ensure_leading_slash to 2 lines.

lib/jekyll/filters/url_filters.rb

@@ -32,13 +32,9 @@ module Jekyll
 
       private
       def ensure_leading_slash(input)
-        return input if input.nil? || input.empty?
+        return input if input.nil? || input.empty? || input.start_with?("/")
-        if input.start_with?("/")
-          input
-        else
         "/#{input}"
       end
-      end
 
     end
   end