SAML2_HTTPRedirect: Fix warning with value-less paramtere.
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.)
tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp