Make spatial merger work with set views

An additional case is needed to make the spatial view merging
possible, which is needed when a cluster is queried.

Change-Id: I6d7bb2092b40f7f879ee32108e5c064c80aef60d
Reviewed-on: http://review.couchbase.org/40134
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>