Fix double-escaping of , and ; in compound properties

I found a bug in the 2.1 version of the library where commas and semicolons get double-escaped in combination of `Property\Compound::setParts()` and `Property::serialize()`. Thus for example ADR properties with commas get wrongly encoded like this:
```
ADR;TYPE=HOME:;;Street\\, Some;City\\, The;;123456;
```

Although I'm not sure if that's the perfect way to fix this issue, it works for me.