__destruct recognized as a command

As with other magic methods like __construct, __destruct shouldn't be recognized as a command.

```
<?php
class MyCommand extends WP_CLI_Command {
   public function __construct() {
      echo "Init...";
   }
   public function __destruct() {
      echo "Hello world";
   }
}

WP_CLI::add_command('test', 'MyCommand');
```
```
 $ wp test __construct
Error: 'test __construct' is not a registered wp command. See 'wp help'.

 $ wp test __destruct 
Init...
Hello world
Hello world
```