<?php
/**
 *
 * This file is part of Relay for PHP.
 *
 * @license http://opensource.org/licenses/MIT MIT
 *
 * @copyright 2015-2018, Paul M. Jones
 *
 */
namespace Relay;

use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Server\MiddlewareInterface;

/**
 *
 * A PSR-15 request handler.
 *
 * @package relay/relay
 *
 */
class Runner extends RequestHandler
{
    /**
     * @inheritdoc
     */
    public function handle(ServerRequestInterface $request) : ResponseInterface
    {
        $entry = current($this->queue);
        $middleware = call_user_func($this->resolver, $entry);
        next($this->queue);

        if ($middleware instanceof MiddlewareInterface) {
            return $middleware->process($request, $this);
        }

        return $middleware($request, $this);
    }
}

<?php
/**
 *
 * This file is part of Relay for PHP.
 *
 * @license http://opensource.org/licenses/MIT MIT
 *
 * @copyright 2015-2018, Paul M. Jones
 *
 */
namespace Relay;

use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

/**
 *
 * A PSR-15 request handler.
 *
 * @package relay/relay
 *
 */
class Runner extends RequestHandler
{
    /**
     * @inheritdoc
     */
    public function handle(ServerRequestInterface $request) : ResponseInterface
    {
        $entry = current($this->queue);
        $middleware = call_user_func($this->resolver, $entry);
        next($this->queue);

        if ($middleware instanceof MiddlewareInterface) {
            return $middleware->process($request, $this);
        }
        
        if ($middleware instanceof RequestHandlerInterface) {
            return $middleware->handle($request);
        }

        return $middleware($request, $this);
    }
}

Support for PSR-15 RequestHandlerInterface for queue items
Support for PSR-15 RequestHandlerInterface for queue items

Problem/details/solution noted here:
https://github.com/relayphp/Relay.Relay/issues/47