#20 Allow parent index OID for walk

[Alex Honeywell]

I have two MIBS, shelfType (indexed by shelfIndex) and slotType (indexed by shelfIndex.slotIndex). When I try to do a walk on slotType.1 (ie. give all slotType's for shelf 1) I get

pysnmp.smi.error.SmiError: Short OID for index slotIndexType()

With the attached diff this error disappears and I get the behaviour I expect.

Is this valid or is there a more proper way of doing this? Doing the equivalent with net-snmp works as expected:

snmpwalk 127.0.0.1 slotType.1

Regards,
- Alex