[mtp] Support initiators that leave mtpParentObject blank

When creating files and directories, Nautilus 3.4.2 was putting
the desired parent object in the request parameters, but not in
the accompanying ObjectInfo. Since the fsstorage plugin took the
parent object from the info, they ended up in the root directory.
Merge pull request #34 from amtep/master

[mtp] Support initiators that leave mtpParentObject blank