Problem was that ha_partition had HA_FILE_BASED flag set
(since it uses a .par file), but after open it uses the first partitions
flags, which results in different case handling for create and for
open.

Solution was to change the underlying partition name so it was consistent.
(Only happens when lower_case_table_names = 2, i.e. Mac OS X and storage
engines without HA_FILE_BASED, like InnoDB and Memory.)

(Recommit after adding rename of check_lowercase_names to
get_canonical_filename, and moved it from handler.h to mysql_priv.h)

NOTE: if a mixed case name for a partitioned table was created when
lower_case_table_name = 2 it should be renamed or dropped before using
the updated version (See bug#37402 for more info)