exclude sitemap calls from tracking (#185)

[stklcode]

resolves #185

The default tracking method does not detect core sitemap calls to wp-sitemap.xml, because there is no check. When WP core sitemaps are disabled, the redirect_template hook is too early, i.e. the sitemap does not yet know if it shall be generated or skipped, so we accidentally track 404 hits in this case.

JS tracking does not seem to be affected, because there either is no wo_footer() for the XML output or is_404() returns true at this point.

Solution: Add a check for the query variable sitemap as suggested in the WP Core discussion¹ to the _is_internal() check, s.t. we never try to track anything for sitemap calls.

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[1] https://make.wordpress.org/core/2020/07/22/new-xml-sitemaps-functionality-in-wordpress-5-5/#comment-39484

[stklcode]

Added another check for the query var sitemap-stylesheet which is set for the XSL call. Don't see any more elegant way at the moment, but it works for me.

[stklcode]

Rebased on top of the latest development and added test case for sitemaps.