For models with composite primary keys, make the Model#primary_key array immutable.
Fix spec in last commit

This spec failed on jruby, rubinius, and ruby 1.8.  Just check that
any error is raised, there's no need to be specific.
Minor MSSQL spec fix

If for some reason the before hook failed half way through, the
after hook would not complete correctly.  This makes sure that
the cleanup still works in that case, and doesn't cause future
runs to fail.