Serializing revisions of atcontent return the current object data for all versions

[pbauer]

While working on collective/collective.exportimport#107 I realize that exprting AT content revisions returns the data of the current object for all versions.

The reason is simple. in atcontent self.context is used instead of obj to get the seriallizer:

serializer = queryMultiAdapter(
    (field, self.context, self.request), IFieldSerializer
)