Load-balanced environments not supported

[vshih]

In SSL-terminating load-balanced environments, bwp-google-xml-sitemaps fails with the following error:

BWP Google XML Sitemaps Error: sitemapindex.xml does not have any item. Please make sure that you have at least one sitemap enabled in BWP Sitemaps >> XML Sitemaps >> Sitemaps to generate.

This is because of a discrepancy in URL scheme:

• Initial sitemap request comes in on HTTPS.
• Load-balancer terminates SSL and forwards request to content server over HTTP.
• WP_HOME is set to HTTPS.
• BWP detects difference in scheme and fails -- BWP_Sitemaps_Sitemap_Provider::is_url_valid() returns false.

I suggest removing the lines following // url must use the current scheme in bwp-google-xml-sitemaps/src/sitemap/provider/provider.php (or reverting d3b58be) to remove this restriction.

Alternate solutions:

• Somehow make use of WordPress' set_url_scheme() to normalize URLs before comparing. We register a filter for this action in order to get WordPress to support our situation.
• Add a hook to allow overriding.
• Make the check optional depending on configuration.
• Add a check to see if $_SERVER['HTTP_X_FORWARDED_PROTO'] is "https" when determining $is_ssl.