Support "Allow" header for 405 errors

src/ErrorFormatter/AbstractFormatter.php

@@ -43,7 +43,13 @@ public function handle(Throwable $error, ServerRequestInterface $request): Respo
         $response = $this->responseFactory->createResponse($this->errorStatus($error));
         $body = $this->streamFactory->createStream($this->format($error, $contentType));
 
-        return $response->withBody($body)->withHeader('Content-Type', $contentType);
+        $response = $response->withBody($body)->withHeader('Content-Type', $contentType);
+
+        if (405 == $response->getStatusCode() && $allow = $this->allow($error)) {
+            $response = $response->withHeader('Allow', implode(', ', $allow));
+        }
+
+        return $response;
     }
 
     protected function errorStatus(Throwable $error): int
@@ -59,6 +65,16 @@ protected function errorStatus(Throwable $error): int
         return 500;
     }
 
+    protected function allow(Throwable $error): ?array
+    {
+        if (method_exists($error, 'getContext')) {
+            $context = $error->getContext();
+            return isset($context['allow']) ? (array) $context['allow'] : null;
+        }
+
+        return null;
+    }
+
     protected function getContentType(ServerRequestInterface $request): ?string
     {
         $accept = $request->getHeaderLine('Accept');

tests/ErrorHandlerTest.php

@@ -56,6 +56,28 @@ public function getStatusCode(): int
         $this->assertEquals(418, $response->getStatusCode());
     }
 
+    public function testNotAllowedException()
+    {
+        $response = Dispatcher::run([
+            new ErrorHandler(),
+            function ($request) {
+                throw new class() extends Exception {
+                    public function getStatusCode(): int
+                    {
+                        return 405;
+                    }
+                    public function getContext(): array
+                    {
+                        return ['allow' => ['GET', 'POST']];
+                    }
+                };
+            },
+        ]);
+
+        $this->assertEquals(405, $response->getStatusCode());
+        $this->assertEquals('GET, POST', $response->getHeaderLine('Allow'));
+    }
+
     public function testGifFormatter()
     {
         $request = Factory::createServerRequest('GET', '/');