Bug #6479  ALTER TABLE ... changing charset fails for TEXT columns
  Fix: use do_conv_blob rather than do_copy_blob
  if the column's character sets are different.


sql/field_conv.cc:
  Bug #6479  ALTER TABLE ... changing charset fails for TEXT columns
  Fix: use do_conv_blob rather than do_copy_blob
  if the column's character sets are different.
mysql-test/t/alter_table.test:
  Bug #6479  ALTER TABLE ... changing charset fails for TEXT columns
  Fix: use do_conv_blob rather than do_copy_blob
  if the column's character sets are different.
mysql-test/r/alter_table.result:
  Bug #6479  ALTER TABLE ... changing charset fails for TEXT columns
  Fix: use do_conv_blob rather than do_copy_blob
  if the column's character sets are different.