item_strfunc.h, item_strfunc.cc, item.cc:
  Try to convert a const item into destination 
  character set. If conversion happens without
  data loss, then cache the converted value
  and return it during val_str().
  Otherwise, if conversion loses data, return
  Illeral mix of collations error, as it happened
  previously.
ctype_recoding.result, ctype_recoding.test:
  Fixing tests accordingly.


sql/item.cc:
  Bug#10446 Illegal mix of collations
  Try to convert a const item into destination 
  character set. If conversion happens without
  data loss, then cache the converted value
  and return it during val_str().
  Otherwise, if conversion loses data, return
  Illeral mix of collations error, as it happened
  previously.
sql/item_strfunc.cc:
  Return cached value when it's possible.
mysql-test/t/ctype_recoding.test:
  Fixing tests accordingly.
mysql-test/r/ctype_recoding.result:
  Fixing tests accordingly.