pt-online-schema-change --dry-run always fails on table with foreign keys

[Harm Verhagen]

pt-online-schema-change --dry-run always fails if a table has a foreign key.
Same command with --execute works.

steps to reproduce
===============
1. Create the following tables
use harm_test;

DROP TABLE IF EXISTS `B`;
DROP TABLE IF EXISTS `A`;
CREATE TABLE `A` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `foo` varchar(30) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

DROP TABLE IF EXISTS `B`;
CREATE TABLE `B` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `a` int(11) NOT NULL,
  KEY `9dde1f34` (`a`),
  PRIMARY KEY (`id`),
  CONSTRAINT `6970ddb42bec57fc` FOREIGN KEY (`a`) REFERENCES `A` (`id`)
) ENGINE=InnoDB;

INSERT INTO `A` VALUES (1,'bar'), (2,'bar2'), (3,'bar3');
INSERT INTO `B` VALUES (1, 1), (2, 2), (3, 1);

2. then run pt-online-schema-change
 pt-online-schema-change --alter "ENGINE=InnoDB" D=harm_test,t=A --user harm --password harm --alter-foreign-keys-method auto --dry-run || echo failed

expected result
============
1. No errors are shown
2. executable exits with a OK return value (0)

actual result
==========
1. you get this error message "Error updating foreign key constraints: Invalid --alter-foreign-keys-method: "
2. executable exits with an ERROR value ( || echo failed -> fires ) making it hard to use it in scripts

{{{
harm@dev3:$ pt-online-schema-change --alter "ENGINE=InnoDB" D=harm_test,t=A --user harm --password harm --alter-foreign-keys-method auto --lock-wait-time=50 --dry-run || echo failed
Child tables:
  `harm_test`.`B` (approx. 3 rows)
Will automatically choose the method to update foreign keys.
Starting a dry run. `harm_test`.`A` will not be altered. Specify --execute instead of --dry-run to alter the table.
Creating new table...
Created new table harm_test._A_new OK.
Altering new table...
Altered `harm_test`.`_A_new` OK.
Not creating triggers because this is a dry run.
Not copying rows because this is a dry run.
Not determining the method to update foreign keys because this is a dry run.
Not swapping tables because this is a dry run.
Error updating foreign key constraints: Invalid --alter-foreign-keys-method:

Not dropping triggers because this is a dry run.
Dropping new table...
Dropped new table OK.
Dry run complete. `harm_test`.`A` was not altered.
failed
}}}

version info
============
mysql 5.1.62-0ubuntu0.10.04.1
percona-toolkit-2.1.1

[Harm Verhagen (harm-verhagen)]

perconatools -> percona-toolkit

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PT-535