Update OpenIDConnectClient.php5

[cicalese]

Allows the caller to send "prompt=login" to cause the authentication server to reauthenticate the end user. The default behavior is unchanged.

[jumbojett]

This is needed functionality but doesn't scale to include additional prompt values. I'd advise adding a generic public function entitled addAuthParam and allow it function similar to addScope.

Auth params could be added in the following way:

$oidc->addAuthParam(array('prompt' => 'login'));