magento/magento2#12310: Fix robots.txt content type to 'text/plain'

app/code/Magento/Robots/Test/Unit/Controller/Index/IndexTest.php

@@ -51,6 +51,9 @@ class IndexTest extends \PHPUnit\Framework\TestCase
         $resultPageMock->expects($this->once())
             ->method('addHandle')
             ->with('robots_index_index');
+        $resultPageMock->expects($this->once())
+            ->method('setHeader')
+            ->with('Content-Type', 'text/plain');
 
         $this->resultPageFactory->expects($this->any())
             ->method('create')