Fix for sitemap issue which caused all pages of a split sitemap to be the same
Test variable before setting (security)
When saving an empty string, the `save_postdata` function would refuse to save, this makes it properly delete the post_meta, which has the effect the user intended with the added benefit of leading to less DB load.
i18n updates

Updated cs_CZ, fa_IR, fr_FR, hu, hu_HU, pl_PL, ru_RU & zh_CN
issue 531 - clean_permalink 301 redirect issue when using https
Merge pull request #532 from pirategaspard/master

issue 531 - clean_permalink 301 redirect issue when using https
Merge git://github.com/jdevalk/wordpress-seo into duplicate-sitemaps

Conflicts:
	readme.txt
Merge pull request #498 from jrfnl/duplicate-sitemaps

Fix for sitemap issue which caused all pages of a split sitemap to be the same
Fixed a bug in the WPSEO tour in WP Network installs

Fixes #537
Changelog Update
Version increment to 1.4.23
Fixed changelog bug and added contributor