Fix Model::isUnique() not working as a validator.

While it *did* work for single fields, isUnique could not be used to
validate the uniqueness across multiple fields as documented. Because
validation methods pass arguments in an order the validator did not
expect the validation method would not work correctly.

Fixes #4571
Merge pull request #4851 from cakephp/issue-4571

Fix Model::isUnique() not working as a validator.