The problem was that when comparing tables for a possible
fast alter table, the comparison was being performed using
the parsed information and not the final definition.
      
The solution is to use the possible final table layout to
compare if a fast alter is possible or not.

mysql-test/include/mix1.inc:
  Disable test case for Bug 21704 as it hasn't been fixed.
mysql-test/r/alter_table.result:
  Add test case result for Bug#33873
mysql-test/r/innodb_mysql.result:
  Update test case result
mysql-test/t/alter_table.test:
  Add test case for Bug#33873
sql/sql_table.cc:
  Use updated (final) information to compare fields.