Fixing problem: validation errors ignored on redirects. Closes #476

In conversation on Issue #476, @lucascs asked me to send this PR to start the bug fix.

Here the gist with the problematic controller as he asked me: [Problematic Controller](https://gist.github.com/renanigt/6f88e6fced50bce47ac9)
@lucascs, what about this: https://github.com/renanigt/vraptor4/blob/validationIgnoredOnRedirects/vraptor-core/src/test/java/br/com/caelum/vraptor/validator/MessagesTest.java#L29 ?