No update for pages built entirely with I18n strings

[komor72]

I just stumbled on an interesting (but minor) problem. I think there is no fundamental way for middleman-search_engine_sitemap to update the sitemap last_mod attribute of a page, that has NOT been modified in its source-code, but the I18n strings used WERE changed. In other words, the situation is: /source/localizable/… is unchanged, but the locales/… are changed and so then build/… is new, but the build/sitemap.xml is not changed.

The workadound is to touch the problematic source-code or just space-delete-save in editor.