MB-8872 check basic auth for _spatial_merge requests

this also requires change in geocouch/gc-couchbase/etc/couchdb/default.d/geocouch.ini
see: http://review.couchbase.org/43429/

Change-Id: I12053e5c8a7c021051eec80bfdabf68978e7deef
Reviewed-on: http://review.couchbase.org/43428
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>