Don't do HEAD request in couch_api_wrap:db_open.

Change-Id: Ie6a0154c9fd2a2d2cafb8b3acb1552bf1c3bca13
Reviewed-on: http://review.couchbase.org/33692
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>