8.2
CS fixes
Add types to private and internal properties
[HttpKernel][Mime][Serializer][String][Validator] Replace `__sleep/wakeup()` by `__(un)serialize()`
[Mime] Reject \Stringable in DataPart::__unserialize()
Merge branch '6.4' into 7.3
Merge branch '7.4' into 8.0
[Mime] Add support for binary Content-Transfer-Encoding