def cache_package
      dir = File.join(page_cache_directory, Jammit.package_path)
      Jammit.packager.cache(@package, @extension, @contents, dir, @variant, @mtime)
    end
def cache_package
      dir = File.join(page_cache_directory, Jammit.package_path)
      Jammit.packager.cache(@package, @extension, @contents, dir, @variant, @mtime)
    end
Tells the Jammit::Packager to cache and gzip an asset package. We can't
 just use the built-in "cache_page" because we need to ensure that
 the timestamp that ends up in the MHTML is also on the cached file.