General: Ensure consistent type for integer properties of WP_Post, WP_Term, and WP_User.

Previously, these properties could be unexpectedly converted to strings in some contexts.

This applies to the following functions:

* `sanitize_post_field()`
* `sanitize_term_field()`
* `sanitize_user_field()`

and the following properties:

* `WP_Post::ID`
* `WP_Post::post_parent`
* `WP_Post::menu_order`
* `WP_Term::term_id`
* `WP_Term::term_taxonomy_id`
* `WP_Term::parent`
* `WP_Term::count`
* `WP_Term::term_group`
* `WP_User::ID`

Props grantmkin, SergeyBiryukov.
Fixes #53235. See #52995.

git-svn-id: https://develop.svn.wordpress.org/trunk@50935 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user.php

@@ -205,6 +205,21 @@ class Tests_User extends WP_UnitTestCase {
 		}
 	}
 
+	/**
+	 * @ticket 53235
+	 */
+	public function test_numeric_properties_should_be_cast_to_ints() {
+		$user     = new WP_User( self::$author_id );
+		$contexts = array( 'raw', 'edit', 'db', 'display', 'attribute', 'js' );
+
+		foreach ( $contexts as $context ) {
+			$user->filter = $context;
+			$user->init( $user->data );
+
+			$this->assertInternalType( 'int', $user->ID );
+		}
+	}
+
 	/**
 	 * Test the magic __unset() method.
 	 *