* src/AthenaOutputStream.cxx: When writing an object with the
exact flag set, it is not sufficient to just cons up a new
DataBucket of the desired type.  The DataProxy is also used to
fill the DataHeaderElement.  So we also make a dummy DataProxy to
go with an AltDataBucket instance.



Former-commit-id: f871f7a148a6903bcfcd53c6eae50a8ed486b50b