adding test case
sql_table.cc:
  sql_table.cc:
  - do not create a new item when charsets are the same
  - return ER_INVALID_DEFAULT if default value cannot
    be converted into the column character set.
item.cc:
  - Allow conversion not only to Unicode,
    but also to and from "binary".
  - Adding safe_charset_converter() for Item_num
    and Item_varbinary, returning a fixed const Item.