Utility of __class__ and PicklingError

Hi,

What is the exact use of https://github.com/collective/collective.alias/blob/master/src/collective/alias/content.py#L259-L274 ? From what I understand, it allow the alias to have the methods/attributes of the aliased, but is it absolutely necessary with the `__getattr__` ?

I'm asking because I have the following error with the plone5 branch et on plone 5 rc2:

```
Traceback (innermost last):
Module ZPublisher.Publish, line 138, in publish
[...]
Module transaction._transaction, line 690, in __init__
Module ZODB.Connection, line 1123, in savepoint
Module ZODB.Connection, line 623, in _commit
Module ZODB.Connection, line 658, in _store_objects
Module ZODB.serialize, line 422, in serialize
Module ZODB.serialize, line 431, in _dump
PicklingError: Can't pickle : it's not the same object as collective.alias.content.Alias
```

Thanks for the help.

Start with src/collective/alias/content.py lines 259-274 and the __class__ and __getattr__ behavior described in the issue. Reproduce the PicklingError on the plone5 branch with Plone 5 RC2, then inspect the ZODB.serialize traceback; done means the alias can be persisted without that error or the necessity of the code is documented.

Written by the indexing model from the issue text.