Add scenario for generating proxies for union types in class properties

[Ocramius]

Similar to #725 and #722, we need to expand our test assets with a class that contains union types for

• methods
• properties
• parameters

For that, we should create a new class alike ClassWithMixedReferenceableTypedProperties:

ProxyManager/tests/ProxyManagerTestAsset/ClassWithMixedReferenceableTypedProperties.php

Lines 1 to 16 in 95f9ccf

	<?php
	declare(strict_types=1);
	namespace ProxyManagerTestAsset;
	/**
	* Base test class to play around with mixed visibility properties with different type definitions
	*
	* @author Marco Pivetta <ocramius@gmail.com>
	* @license MIT
	*/
	class ClassWithMixedReferenceableTypedProperties
	{
	public static $publicStaticUnTypedProperty = 'publicStaticUnTypedProperty';
	public static $publicStaticUnTypedPropertyWithoutDefaultValue;

Specifically, |null needs very careful evaluation, as nullable vs non-nullable union types have quite sensitive semantics around code generation and by-ref evaluation.

Once added, we should use this new class wherever ClassWithMixedReferenceableTypedProperties (and proceed fixing any potentially detected issues).