Support "redirect_to" parameter on login page

includes/openid-connect-generic-login-form.php

@@ -89,10 +89,14 @@ class OpenID_Connect_Generic_Login_Form {
 		// record the URL of this page if set to redirect back to origin page
 		if( $this->settings->redirect_user_back ) {
 			$redirect_expiry = time() + DAY_IN_SECONDS;
-			if ( $GLOBALS['pagenow'] == 'wp-login.php' )
-				$redirect_url = admin_url();
-			else
+			if ( $GLOBALS['pagenow'] == 'wp-login.php' ) {
+				if( isset( $_REQUEST['redirect_to'] ) )
+					$redirect_url = esc_url( $_REQUEST['redirect_to'] );
+				else
+					$redirect_url = admin_url();
+			} else {
 				$redirect_url = home_url( esc_url( add_query_arg( NULL, NULL ) ) );
+			}
 			setcookie( $this->client_wrapper->cookie_redirect_key, $redirect_url, $redirect_expiry, COOKIEPATH, COOKIE_DOMAIN, is_ssl() );
 		}