MB-8582: Make couch_set_view:fold/5 indexer agnostic

The `ViewQueryArgs` in couch_set_view:fold/5 is a
but a #spatial_query_args{} record in the spatial views.
With this change the function doesn't care which type
it has, but just passed on the value.

Change-Id: Ie5986cf554b5b7cb7dd215991097aa2158d95585
Reviewed-on: http://review.couchbase.org/27845
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>