Multisite: Improve caching in WP_Site_Query by ignoring the $fields argument.

Prior to this change there were two different cache keys used for the same query. That is because regardless of the `$fields` argument, the query response will be the same.

Props spacedmonkey.
Fixes #41197.

Built from https://develop.svn.wordpress.org/trunk@41059


git-svn-id: http://core.svn.wordpress.org/trunk@40909 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-site-query.php

@@ -251,7 +251,12 @@ class WP_Site_Query {
 		do_action_ref_array( 'pre_get_sites', array( &$this ) );
 
 		// $args can include anything. Only use the args defined in the query_var_defaults to compute the key.
-		$key = md5( serialize( wp_array_slice_assoc( $this->query_vars, array_keys( $this->query_var_defaults ) ) ) );
+		$_args = wp_array_slice_assoc( $this->query_vars, array_keys( $this->query_var_defaults ) );
+
+		// Ignore the $fields argument as the queried result will be the same regardless.
+		unset( $_args['fields'] );
+
+		$key = md5( serialize( $_args ) );
 		$last_changed = wp_cache_get_last_changed( 'sites' );
 
 		$cache_key = "get_sites:$key:$last_changed";

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41058';
+$wp_version = '4.9-alpha-41059';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.