Strip slashes on nil url tokens

lib/jekyll/url.rb

@@ -61,9 +61,14 @@ module Jekyll
     def generate_url(template)
       @placeholders.inject(template) do |result, token|
         break result if result.index(':').nil?
+        if token.last.nil?
+          # Remove leading '/' to avoid generating urls with `//`
+          result.gsub(/\/:#{token.first}/, '')
+        else
           result.gsub(/:#{token.first}/, self.class.escape_path(token.last))
         end
       end
+    end
 
     # Returns a sanitized String URL
     def sanitize_url(in_url)