Resolve Redirect: Explicitly pass the parameter to be validated (#14538)

In certain situations a second parameter was being passed to the
`validate_callback` of the resolve-redirect endpoint. This was causing
an exception depending on its value.

This change wraps the callback to ensure that only one parameter is
passed.

_inc/lib/core-api/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-resolve-redirect.php

@@ -36,7 +36,9 @@ public function register_routes() {
 						'description'       => __( 'The URL to check for redirects.', 'jetpack' ),
 						'type'              => 'string',
 						'required'          => 'true',
-						'validate_callback' => 'wp_http_validate_url',
+						'validate_callback' => function ( $param ) {
+							return wp_http_validate_url( $param );
+						},
 					),
 				),
 				array(