PERL-726 Update deprecated 'last_error' test
PERL-726 Update deprecated 'last_error' test

mongos from MongoDB 3.5.2 appears to now return a defined 'n:0' value when
previous mongos omitted 'n'.  The test now only checks for a false value.

mongodb/mongo-perl-driver,dagolden/mongo-perl-driver,dagolden/mongo-perl-driver,dagolden/mongo-perl-driver,mongodb/mongo-perl-driver,xdg/mongo-perl-driver,xdg/mongo-perl-driver,xdg/mongo-perl-driver,dagolden/mongo-perl-driver,mongodb/mongo-perl-driver