Refactor discriminator merging for Scrutinizer

https://scrutinizer-ci.com/g/schmittjoh/serializer/inspections/36942cd9-00d8-4262-917c-8738dc7f359a/code-structure/operation/JMS%5CSerializer%5CMetadata%5CClassMetadata%3A%3Amerge
Merge branch 'fix/discriminator-merging' into feat/discriminator-group

Conflicts:
	src/JMS/Serializer/Metadata/ClassMetadata.php