Fix WebApi test to be compatible with php8

Author: anzin

dev/tests/api-functional/framework/Magento/TestFramework/Authentication/Rest/CurlClient.php

@@ -24,4 +24,17 @@ public function retrieveResponse(
         $this->setCurlParameters([CURLOPT_FAILONERROR => true]);
         return parent::retrieveResponse($endpoint, $requestBody, $extraHeaders, $method);
     }
+
+    /**
+     * @inheritdoc
+     */
+    public function normalizeHeaders(&$headers)
+    {
+        array_walk(
+            $headers,
+            function (&$val, $key) {
+                $val = ucfirst(strtolower($key)) . ': ' . $val;
+            }
+        );
+    }
 }