Added backwards compatibility layer (#133)

Author: Nyholm

src/Plugin/VersionBridgePlugin.php

@@ -4,6 +4,7 @@
 
 namespace Http\Client\Common\Plugin;
 
+use Http\Promise\Promise;
 use Psr\Http\Message\RequestInterface;
 
 /**
@@ -16,7 +17,7 @@ trait VersionBridgePlugin
 {
     abstract protected function doHandleRequest(RequestInterface $request, callable $next, callable $first);
 
-    public function handleRequest(RequestInterface $request, callable $next, callable $first)
+    public function handleRequest(RequestInterface $request, callable $next, callable $first): Promise
     {
         return $this->doHandleRequest($request, $next, $first);
     }

src/VersionBridgeClient.php

@@ -5,6 +5,7 @@
 namespace Http\Client\Common;
 
 use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
 
 /**
  * A client that helps you migrate from php-http/httplug 1.x to 2.x. This
@@ -16,7 +17,7 @@ trait VersionBridgeClient
 {
     abstract protected function doSendRequest(RequestInterface $request);
 
-    public function sendRequest(RequestInterface $request)
+    public function sendRequest(RequestInterface $request): ResponseInterface
     {
         return $this->doSendRequest($request);
     }