Fixes bug when using nodegroups assigned to SliceAttributes:

Fixes bug when using nodegroups assigned to SliceAttributes:

    File "/usr/share/plc_api/PLC/Methods/GetSlivers.py", line 219, in call
	        slivers = get_slivers(self.api, slice_ids, node)
	File "/usr/share/plc_api/PLC/Methods/GetSlivers.py", line 82, in get_slivers
		if slice_attribute['name'] not in slice_attributes:
    File "/usr/share/plc_api/PLC/Table.py", line 209, in __eq__
		y = self.db_fields(y)
	File "/usr/share/plc_api/PLC/Table.py", line 198, in db_fields
		obj.items()))
	AttributeError: 'unicode' object has no attribute 'items'