Fixes a warning that gets emitted when mocking AMQPChannel with some methods

In our test suite, we are mocking the `AMQPChannel` class to emulate some RabbitMQ parts of our application, but we started seeing this when upgrading to 2.4.0:

```
Undefined property: Mock_AMQPChannel_4c2e1e56::$is_open

/home/ptd/vendor/videlalvaro/php-amqplib/PhpAmqpLib/Channel/AMQPChannel.php:155
/home/ptd/vendor/videlalvaro/php-amqplib/PhpAmqpLib/Channel/AMQPChannel.php:109
/usr/share/php/PHPUnit/TextUI/Command.php:176
/usr/share/php/PHPUnit/TextUI/Command.php:129
```

(note that this is from *our* test suite). This is because the `$is_open` variable is never set, so it emits this warning, which is failing our test suite (because it gets treated as an error it seems).

This change will fix this issue, and I have included a unit test which proves the fix. An alternate solution is to define `$this->is_open` as a property in the class. Please let me know if this is preferable.

For reference, [this](https://github.com/videlalvaro/php-amqplib/commit/9226d61abb63f7d7e8d7094d2c30d510cfe2c9f8#diff-5392acd70f1025abd651d60a8a8007c4R156) (Line 156) is the line introduced between 2.3.0 and 2.4.0 that broke our tests :)