Fix bug in miner extend sector expiration

[austinabell]

Summary of changes
Changes introduced in this pull request:

• related to v7 upgrade, I misread the go logic and didn't realize there was an implicit inserting of a default. Logic copied here:

					prevEpochPartitions, ok := partitionsByNewEpoch[decl.NewExpiration]
					partitionsByNewEpoch[decl.NewExpiration] = append(prevEpochPartitions, decl.Partition)
					if !ok {
						epochsToReschedule = append(epochsToReschedule, decl.NewExpiration)
					}

Also the updating of sectors and deadlines was not updated, and I'm not sure how this wasn't hit before.

Reference issue to close (if applicable)

Closes

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