Aupajo/middleman-search_engine_sitemap — pull request #3 by createdbypete (2014-09-15 21:36:06+00:00)
Title: Check resource has a source file

I recently found that using this extension with [middleman-alias](https://github.com/Octo-Labs/middleman-alias) will cause an error because these resources have a `nil` value returned from `source_file` so the sitemap cannot establish the `File.mtime`.

This proposed change simply checks that `source_file` is not `nil`.