Mutli-table uses temporary table to store new values for fields. With the
new values the rowid of the record to be updated is stored in a Field_string
field. Table to be updated is set as source table of the rowid field.
But when the temporary table creates the tmp field for the rowid field it
converts it to a varstring field because the table to be updated was created by
the v4.1. Due to this the stored rowids were broken and no records for 
update were found.

The flag can_alter_field_type is added to Field_string class. When it is set to
0 the field won't be converted to varstring. The Field_string::type() function 
now always returns MYSQL_TYPE_STRING if can_alter_field_type is set to 0.
The multi_update::initialize_tables() function now sets can_alter_field_type
flag to 0 for the rowid fields denying conversion of the field to a varstring
field.