WP: Don't attempt to convert multiple-nested arrays to a string in WP->parse_request().

See #14330.
Fixes #42752.


git-svn-id: https://develop.svn.wordpress.org/trunk@42419 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp.php

@@ -308,7 +308,7 @@ class WP {
 					$this->query_vars[ $wpvar ] = (string) $this->query_vars[ $wpvar ];
 				} else {
 					foreach ( $this->query_vars[ $wpvar ] as $vkey => $v ) {
-						if ( ! is_object( $v ) ) {
+						if ( is_scalar( $v ) ) {
 							$this->query_vars[ $wpvar ][ $vkey ] = (string) $v;
 						}
 					}