Fixed test cases (#7643)

Author: limingxinleo

src/Cookie/Cookie.php

@@ -94,7 +94,7 @@ public function __construct(
      *
      * @return string The cookie
      */
-    public function __toString()
+    public function __toString(): string
     {
         $str = ($this->isRaw() ? $this->getName() : urlencode($this->getName())) . '=';
 

src/Cookie/SetCookie.php

@@ -52,7 +52,7 @@ public function __construct(array $data = [])
         }
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         $str = $this->data['Name'] . '=' . $this->data['Value'] . '; ';
         foreach ($this->data as $k => $v) {

src/Stream/StandardStream.php

@@ -16,6 +16,7 @@
 use InvalidArgumentException;
 use Psr\Http\Message\StreamInterface;
 use RuntimeException;
+use Stringable;
 
 use function clearstatcache;
 use function fclose;
@@ -42,7 +43,7 @@
  * Author: Martijn van der Ven <martijn@vanderven.se>.
  * @license https://github.com/Nyholm/psr7/blob/master/LICENSE
  */
-final class StandardStream implements StreamInterface
+final class StandardStream implements Stringable, StreamInterface
 {
     /** @var array Hash of readable and writable stream types */
     private const READ_WRITE_HASH = [