[Mailer][Mime] Fix case-sensitive handling of header names

Author: piku235

EventListener/MessageListener.php

@@ -58,7 +58,7 @@ public function addHeaderRule(string $headerName, int $rule): void
             throw new InvalidArgumentException(sprintf('The "%d" rule is not supported.', $rule));
         }
 
-        $this->headerRules[$headerName] = $rule;
+        $this->headerRules[strtolower($headerName)] = $rule;
     }
 
     public function onMessage(MessageEvent $event): void

Tests/EventListener/MessageListenerTest.php

@@ -101,5 +101,17 @@ public function provideHeaders(): iterable
             ->add(new MailboxListHeader('bcc', [new Address('bcc-initial@example.com'), new Address('bcc-default@example.com'), new Address('bcc-default-1@example.com')]))
         ;
         yield 'bcc, add another bcc (unique header)' => [$initialHeaders, $defaultHeaders, $expectedHeaders];
+
+        $initialHeaders = (new Headers())
+            ->add(new UnstructuredHeader('foo', 'initial'))
+        ;
+        $defaultHeaders = (new Headers())
+            ->add(new UnstructuredHeader('foo', 'bar'))
+            ->add(new UnstructuredHeader('bar', 'foo'))
+        ;
+        $rules = [
+            'Foo' => MessageListener::HEADER_REPLACE,
+        ];
+        yield 'Capitalized header rule (case-insensitive), replace if set' => [$initialHeaders, $defaultHeaders, $defaultHeaders, $rules];
     }
 }