SAML2_HTTPRedirect: Fix warning with value-less paramtere.

This patch fixes warnings from query strings like:

    http://.../endpoint.php?foo&bar=test

(Where the parameter "foo" doesn't have a value.)

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3197 44740490-163a-0410-bde0-09ae8108e29a