added an assignment operator to ShortString<> taking std::string
added an assignment operator to ShortString<> taking std::string

djw8605/cvmfs,trshaffer/cvmfs,MicBrain/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,cvmfs-testing/cvmfs,cvmfs-testing/cvmfs,Moliholy/cvmfs,cvmfs/cvmfs,reneme/cvmfs,reneme/cvmfs,MicBrain/cvmfs,alhowaidi/cvmfsNDN,Gangbiao/cvmfs,trshaffer/cvmfs,cvmfs-testing/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,Moliholy/cvmfs,Moliholy/cvmfs,reneme/cvmfs,alhowaidi/cvmfsNDN,alhowaidi/cvmfsNDN,trshaffer/cvmfs,trshaffer/cvmfs,bbockelm/cvmfs,Moliholy/cvmfs,cvmfs/cvmfs,cvmfs-testing/cvmfs,MicBrain/cvmfs,cvmfs/cvmfs,Moliholy/cvmfs,cvmfs/cvmfs,MicBrain/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,djw8605/cvmfs,djw8605/cvmfs,reneme/cvmfs,bbockelm/cvmfs,trshaffer/cvmfs,Gangbiao/cvmfs,cvmfs-testing/cvmfs,MicBrain/cvmfs,bbockelm/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,alhowaidi/cvmfsNDN,reneme/cvmfs,bbockelm/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,alhowaidi/cvmfsNDN,djw8605/cvmfs,bbockelm/cvmfs,Gangbiao/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs