fix(Webapi Xml Renderer - 18361): added unit test for the issue

Author: nickshatilo

lib/internal/Magento/Framework/Webapi/Rest/Response/Renderer/Xml.php

@@ -111,7 +111,7 @@ protected function _formatValue($value)
             /** Without the following transformation boolean values are rendered incorrectly */
             $value = $value ? 'true' : 'false';
         }
-        return $value;
+        return (string) $value;
     }
 
     /**

lib/internal/Magento/Framework/Webapi/Test/Unit/Rest/Response/Renderer/XmlTest.php

@@ -76,6 +76,11 @@ public function providerXmlRender()
                 '<?xml version="1.0"?><response><item_7key>value</item_7key></response>',
                 'Invalid XML render with numeric symbol in data index.'
             ],
+            [
+                ['key' => 'test & foo'],
+                '<?xml version="1.0"?><response><key>test &amp; foo</key></response>',
+                'Invalid XML render with ampersand symbol in data index.'
+            ],
             [
                 ['.key' => 'value'],
                 '<?xml version="1.0"?><response><item_key>value</item_key></response>',