Feature/custom http request headers (#605)

* include support for custom http request headers

* format headers key/value pairs

* Update CurlClient.php linting

* remove lingering whitespace

src/Adyen/HttpClient/CurlClient.php

@@ -306,6 +306,13 @@ public function requestHttp(Service $service, $requestUrl, $params, $method, $re
             $libraryVersion
         );
 
+        // If headers are provided in requestOptions include them in the request
+        if (!empty($requestOptions['headers'])) {
+            foreach ($requestOptions['headers'] as $headerKey => $headerValue) {
+                $headers[] = $headerKey . ': ' . $headerValue;
+            }
+        }
+
         // If idempotency key is provided as option include into request
         if (!empty($requestOptions['idempotencyKey'])) {
             $headers[] = 'Idempotency-Key: ' . $requestOptions['idempotencyKey'];