"dump_dir" should be optional when creating serializers if "fh" is given

[Natalia Bidart]

Current initialization for OOPSFileSerializer is:

    def __init__(self, key, dump_dir, fh=None):
        super(OOPSFileSerializer, self).__init__(key)
        self._dump_dir = dump_dir
        self._filename = None
        self._oops = None
        self._fh = fh
        if not os.path.isdir(dump_dir):
            os.makedirs(dump_dir)

if fh is defined, the serializer shouldn't require a mandatory "dump_dir" and it shouldn't create the dir either.