Problem was with handling NULL values in ranges


mysql-test/r/partition_hash.result:
  New partition pruning test cases
mysql-test/r/partition_list.result:
  New partition pruning test cases
mysql-test/r/partition_pruning.result:
  New partition pruning test cases
mysql-test/r/partition_range.result:
  New partition pruning test cases
mysql-test/t/partition_hash.test:
  New partition pruning test cases
mysql-test/t/partition_list.test:
  New partition pruning test cases
mysql-test/t/partition_pruning.test:
  New partition pruning test cases
mysql-test/t/partition_range.test:
  New partition pruning test cases
sql/opt_range.cc:
  Added comment
sql/sql_partition.cc:
  Partition pruning didn't handle ranges with NULL values in a proper manner