Convert MCFakeFile lfn from unicode to string

As reported on this elog:
https://cms-logbook.cern.ch/elog/Workflow+processing/17434

WorkQueueManager is complaining in all the MC agents (but not reproc). 
Since the try/except block encapsulates lots of calls, the debugging was a bit tricky but in the end it maps to this call
https://github.com/dmwm/WMCore/blob/master/src/python/WMCore/WMBS/File.py#L619
which makes this SQL call
https://github.com/dmwm/WMCore/blob/master/src/python/WMCore/WMBS/MySQL/Files/AddRunLumi.py#L27

Maybe this convertion should be done in the latter file instead?