[BUGFIX] Support serialization of classes with static properties
When trying to serialize a proxied class that contains static
properties an exception is thrown::
Runtime Notice: Accessing static property [...] as non static
The reason is that the ``DependencyInjection\\ProxyClassBuilder``
overrides the magic ``__sleep()`` method of the proxy class, accessing
all properties as ``$this->\$propertyName``.
This change fixes the issue by skipping static properties.
Releases: master, 3.0, 2.3, 2.2, 2.1