When partition pruning resulted in an ordered index scan spanning only
one partition, any descending flag for the scan was wrongly discarded,
turning ORDER BY DESC into ORDER BY ASC, and similar problems.

Fixed by correctly passing descending flag in SCAN_TABREQ signal sent
to data nodes.


mysql-test/suite/ndb/r/ndb_partition_key.result:
  Test case.
mysql-test/suite/ndb/r/ndb_partition_range.result:
  Test case.
mysql-test/suite/ndb/t/ndb_partition_key.test:
  Test case.
mysql-test/suite/ndb/t/ndb_partition_range.test:
  Test case.
storage/ndb/src/ndbapi/NdbScanOperation.cpp:
  Even if ordered scan not requested (no need to merge-sorts scans of
  each partition), we still need to honor the descending flag.