Create temporary InnoDB table fails on case insensitive
filesystems, when lower_case_table_names is 2 (e.g. OS X)
and temporary directory path contains upper case letters.

The problem was that tmpdir prefix was converted to lower
case when table was created, but was passed as is when
table was opened.

Fixed by leaving tmpdir prefix part intact.

mysql-test/r/lowercase_mixed_tmpdir_innodb.result:
  A test case for BUG#45638.
mysql-test/t/lowercase_mixed_tmpdir_innodb-master.opt:
  A test case for BUG#45638.
mysql-test/t/lowercase_mixed_tmpdir_innodb-master.sh:
  A test case for BUG#45638.
mysql-test/t/lowercase_mixed_tmpdir_innodb.test:
  A test case for BUG#45638.
sql/handler.cc:
  Fixed get_canonical_filename() to not lowercase filesystem
  path prefix for temporary tables.