differences in tables
Certain merge tables were wrongly reported as having incorrect definition:
- Some fields that are 1 byte long (e.g. TINYINT, CHAR(1)), might
  be internally casted (in certain cases) to a different type on a
  storage engine layer. (affects 4.1 and up)
- If tables in a merge (and a MERGE table itself) had short VARCHAR column (less
  than 4 bytes) and at least one (but not all) tables were ALTER'ed (even to an
  identical table: ALTER TABLE xxx ENGINE=yyy), table definitions went ouf of
  sync. (affects 4.1 only)

This is fixed by relaxing a check for underlying conformance and setting
field type to FIELD_TYPE_STRING in case varchar is shorter than 4
when a table is created.