Move System::httpExit to BaseModule->httpExit

- This will ensure headers set in BaseModule->run will be carried in httpExit scenarios
- Deprecate httpExit() method in Core\System

src/Module/Response.php

@@ -89,7 +89,7 @@ class Response implements ICanCreateResponses
 	/**
 	 * {@inheritDoc}
 	 */
-	public function setType(string $type, ?string $content_type = null): void
+	public function setType(string $type = Response::TYPE_HTML, ?string $content_type = null): void
 	{
 		if (!in_array($type, static::ALLOWED_TYPES)) {
 			throw new InternalServerErrorException('wrong type');