Bug 163039, relax sanity check on XML serializer so that we won't throw an error if the serialized document is not XML. This is needed because Bugzilla (at least) abuses XMLSerializer (to edit attachments). r=sicking, sr=jst.

content/base/src/nsXMLContentSerializer.cpp

@@ -764,7 +764,9 @@ nsXMLContentSerializer::AppendDocumentStart(nsIDOMDocument *aDocument,
   NS_ENSURE_ARG_POINTER(aDocument);
 
   nsCOMPtr<nsIXMLDocument> xml(do_QueryInterface(aDocument));
-  NS_ENSURE_TRUE(xml, NS_ERROR_UNEXPECTED);
+  if (!xml) {
+    return NS_OK;
+  }
 
   nsAutoString version, encoding, standalone;
   xml->GetXMLDeclaration(version, encoding, standalone);