workaround for CERN xrootd bug

The new xrootd version used at CERN for SLC6 (3.3.1) has a bug in the feature that allows to pass size and checksum metadata. Implement it another way.
Merge pull request #4604 from hufnagel/cernxrootdbug

workaround for checksums at CERN, xrootd bug