Proxies shouldn't serialize static properties in __sleep()

This PR contains a test and a fix for the following bug: Proxies did serialize static properties.

I believe this is a regression in 2.4 since I've never met this bug before.

Given the class:

```php
class StaticPropertyClass
{
    protected static $protectedStaticProperty;
}
```

Before the fix, proxies would contain the following `__sleep` method:

```php
    public function __sleep()
    {
        if ($this->__isInitialized__) {
            return array('__isInitialized__', 'protectedStaticProperty');
        }

        return array('__isInitialized__', 'protectedStaticProperty');
    }
```

With the fix:

```php
    public function __sleep()
    {
        if ($this->__isInitialized__) {
            return array('__isInitialized__');
        }

        return array('__isInitialized__');
    }
```