Explicitly specify status code 200 as response code

Potentially fixes https://github.com/owncloud/core/issues/17586

lib/private/connector/sabre/dummygetresponseplugin.php

@@ -61,6 +61,7 @@ class DummyGetResponsePlugin extends \Sabre\DAV\ServerPlugin {
 		fwrite($stream, $string);
 		rewind($stream);
 
+		$response->setStatus(200);
 		$response->setBody($stream);
 
 		return false;

tests/lib/connector/sabre/DummyGetResponsePluginTest.php

@@ -59,6 +59,10 @@ class DummyGetResponsePluginTest extends TestCase {
 		$response
 			->expects($this->once())
 			->method('setBody');
+		$response
+			->expects($this->once())
+			->method('setStatus')
+			->with(200);
 
 		$this->assertSame(false, $this->dummyGetResponsePlugin->httpGet($request, $response));
 	}