Fix issue #1073 date localization

Fixes dates not always being localized in the `wpseo_replace_vars()` function.

Also:
* similar date localization in metabox preview snippet (after all Google is localized too)
* minor efficiency tweak in `wpseo_replace_vars()`
* ensure that dates in sitemap are *not* localized - this might fix some invalid date format issues people are having when validation the sitemap in GWT