REST API: Add author, modified, and parent sort order options for posts.

These (and a few others that can be revisited later if needed) were present in
beta versions of the WP REST API but were removed during the merge to WP 4.7.

Props ChopinBach, jnylen0.
Fixes #38693.


git-svn-id: https://develop.svn.wordpress.org/trunk@40605 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/testcase.php

@@ -868,4 +868,28 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 		wp_update_attachment_metadata( $id, wp_generate_attachment_metadata( $id, $upload['file'] ) );
 		return $id;
 	}
+
+	/**
+	 * There's no way to change post_modified through WP functions.
+	 */
+	protected function update_post_modified( $post_id, $date ) {
+		global $wpdb;
+		return $wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_modified' => $date,
+				'post_modified_gmt' => $date,
+			),
+			array(
+				'ID' => $post_id,
+			),
+			array(
+				'%s',
+				'%s',
+			),
+			array(
+				'%d',
+			)
+		);
+	}
 }