Support PHP 8.1 (#4333)

Author: limingxinleo

src/Cookie/CookieJar.php

@@ -13,6 +13,7 @@
 
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ResponseInterface;
+use Traversable;
 
 /**
  * Cookie jar that stores cookies as an array.
@@ -208,12 +209,12 @@ public function setCookie(SetCookie $cookie)
         return true;
     }
 
-    public function count()
+    public function count(): int
     {
         return count($this->cookies);
     }
 
-    public function getIterator()
+    public function getIterator(): Traversable
     {
         return new \ArrayIterator(array_values($this->cookies));
     }

src/Upload/UploadedFile.php

@@ -69,11 +69,11 @@ public function __toString()
     /**
      * TODO: ?string => string.
      */
-    public function getExtension(): ?string
+    public function getExtension(): string
     {
         $clientName = $this->getClientFilename();
         $segments = explode('.', $clientName);
-        return end($segments);
+        return (string) end($segments);
     }
 
     public function getMimeType(): string