Fix error when reading classes with a mixed property

[jorrit]

I created an object with a property with type 'mixed', as is possible since PHP 8.0.

Unfortunately, this doesn't work, despite various checks in the code for $type === 'mixed'. It malfunctions in my case when mixed is not used as an annotation. but as a PHP property type. Around line 571 a namespace prefix is added because mixed is not recognized as a simple type.

This PR fixes that and adds a test.