saml:SP: Handle duplicate response.

In some cases, we may receive duplicate responses when the user uses
the back-button in the browser to go back to the HTTP-POST page. This
patch changes the behaviour to just redirect the user to the correct
page, instead of displaying a confusing error message.

Fixes issue 404.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2873 44740490-163a-0410-bde0-09ae8108e29a