Add support for multi-value response headers with API Gateway v2

src/Event/Http/HttpResponse.php

@@ -72,7 +72,7 @@ public function toApiGatewayFormatV2(): array
             } else {
                 // Make sure the values are never arrays
                 // because API Gateway v2 does not support multi-value headers
-                $headers[$name] = is_array($values) ? end($values) : $values;
+                $headers[$name] = is_array($values) ? implode(', ', $values) : $values;
             }
         }
 

tests/Event/Http/HttpResponseTest.php

@@ -58,7 +58,7 @@ public function test headers are capitalized()
         ], $response->toApiGatewayFormatV2());
     }
 
-    public function test nested arrays in headers are flattened()
+    public function test multi value headers()
     {
         $response = new HttpResponse('', [
             'foo' => ['bar', 'baz'],
@@ -76,8 +76,10 @@ public function test nested arrays in headers are flattened()
             'cookies' => [],
             'isBase64Encoded' => false,
             'statusCode' => 200,
-            // The last value is kept (when multiheaders are not enabled)
-            'headers' => ['Foo' => 'baz'],
+            // Headers are joined with a comma
+            // See https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.2
+            // API Gateway v2 does not support multi-value headers
+            'headers' => ['Foo' => 'bar, baz'],
             'body' => '',
         ], $response->toApiGatewayFormatV2());
     }