Memcached::getAllKeys() Failure on 3.21+

**HHVM Version**: 3.21 (and above)

The `getAllKeys()` method for the `Memcached` library is failing with a general client error. This method was working in previous versions of HHVM but is no longer working on versions 3.21 and above.

**Example Code:**
```
$mc = new Memcached();
$mc->addServer('127.0.0.1','11211');
$mc->set('example_key', 'example_value', 0);
$all_keys = $mc->getAllKeys();
print_r($all_keys);
```

**Expected Result:**
```
Array
(
[0] => example_key
)
```

**Actual Result:**
```
false
```

If you read the Memcached response code (`getResultCode()`), you will see the code is `09` or textually: `MEMCACHED_CLIENT_ERROR`.

**Additional Details:**
- Memcache Server Version: 1.4.25
- Memcached PHP Module Version: 2.2.0b1