Fix header writing

src/Server.php

@@ -88,11 +88,12 @@ class Server
 
         header('HTTP/' . $response->getProtocolVersion() . ' ' . $response->getStatusCode() . ' ' . $response->getReasonPhrase());
 
-        $wrote = [];
-        foreach ($response->getHeaders() as [$headerName, $headerValue]) {
-            // Only replace header if we didn't write on those yet
-            header($headerName . ': ' . $headerValue, !isset($wrote[$headerName]));
-            $wrote[$headerName] = true;
+        foreach ($response->getHeaders() as $headerName => $headerValues) {
+            $first = true;
+            foreach ($headerValues as $headerValue) {
+                header($headerName . ': ' . $headerValue, $first === true);
+                $first = false;
+            }
         }
 
         $stdout = fopen('php://output', 'w+');