Properly handle TStreamerInfo object in dqm-access python script.

This will allow the user to donwload locally any ROOT object from the DQM GUI servers,
without bothering about which ROOT version was used to register the original
ROOT files into the servers and which version he is currently using via CMSSW.
This was tested in a real case of schema evolution of a TH1 object in going
from ROOT 5.32 (used by DQMGUI) and ROOT 5.34, used by any recent CMSSW.