[FIX] Plugin breaks single templates

Plugin: https://www.ads-software.com/plugin/google-authorship-for-multiple-writers

To fix replace following code:

add_action('single_template', 'google_authorship_single');
function google_authorship_single() {
	global $google_authorship_single;
	$google_authorship_single = true;
}

with:

add_filter( 'single_template', 'google_authorship_single', 10, 1 );
function google_authorship_single( $template ) {
	global $google_authorship_single;
	$google_authorship_single = true;

	return $template;
}

But the best would be to replace usage of global $google_authorship_single with conditional tag is_singular() in functions that needs this information.

Why?! ‘single_template’ is a filter hook and not an action hook – so, it must return value

https://www.ads-software.com/plugins/google-authorship-for-multiple-writers/