Apart from the fix, the patch also adds few more unrelated test
cases for partial matching, and fixes few typos.

Analysis:
This bug uncovered that partial matching via rowid intersection
didn't handle the case when:
- the left IN argument has some NULLs,
- there are no non-null value matches, and there is no non-null
  column,
- the subquery columns that are not covered with the NULLs in
  the left IN argument contain at least one row, such that it
  has NULL values in all columns where the left IN operand has
  no NULLs.
In this case there is a partial match.

In addition the analysis of the related code uncovered incorrect
handling of few other related cases.

Solution:
The solution for the bug is to check if there exists a row with
NULLs in all columns other than the ones having NULL in the
let IN operand.

The check is implemented via checking whether the bitmaps that
store NULL information in class Ordered_key have a non-empty
intersection for the relevant columns.

The intersection itself is implemented via the function
bitmap_exists_intersection() in my_bitmap.c.