cubicweb #2137793 BFSS can try to create a Binary with a unicode object [validation pending]

this can happen if the root directory passed to the constructor is a unicode string. It looks like:

 File "/home/alf/cubicweb/cubicweb/server/sources/native.py", line 636, in add_entity
   with self._storage_handler(entity, 'added'):
 File "/usr/lib/python2.6/contextlib.py", line 16, in __enter__
   return self.gen.next()
 File "/home/alf/cubicweb/cubicweb/server/sources/native.py", line 625, in _storage_handler
   to_restore = handler(entity, attr)
 File "/home/alf/cubicweb/cubicweb/server/sources/storages.py", line 138, in entity_added
   entity.cw_edited.edited_attribute(attr, Binary(fpath))
 File "/home/alf/cubicweb/cubicweb/__init__.py", line 72, in __init__
   "Binary objects must use raw strings, not %s" % buf.__class__
AssertionError: Binary objects must use raw strings, not <type 'unicode'>
priorityimportant
typebug
done in3.14.3
load0.100
load left0.000
closed by#b7a195d54fd4 [bfss] convert root BFSS directory to str in constructor (closes #2137793)