Accept 'ID' as a valid $field in get_user_by().

We already accept 'id'. `ID` more closely matches the database and
`WP_User` schemas.

Props Shelob9.
Fixes #33869.

git-svn-id: https://develop.svn.wordpress.org/trunk@34125 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user.php

@@ -445,6 +445,16 @@ class Tests_User extends WP_UnitTestCase {
 		$this->assertEquals( false, $user );
 	}
 
+	/**
+	 * @ticket 33869
+	 */
+	public function test_user_get_data_by_ID_should_alias_to_id() {
+		$u = $this->factory->user->create();
+
+		$user = WP_User::get_data_by( 'ID', $u );
+		$this->assertEquals( $u, $user->ID );
+	}
+
 	/**
 	 * @ticket 21431
 	 */