fix(xo-server/xo.exportConfig): dont return 200 on failure

`writeHead` sends immediatly the response header, which has a number of undesirable side-effects.

For instance, in this case, it prevented returning a 500 code in case of error.

It's almost always preferable to use `statusCode`, `writeHead()` or Express methods like `set()`.

packages/xo-server/src/api/xo.js

@@ -18,7 +18,7 @@ export async function exportConfig() {
   return {
     $getFrom: await this.registerHttpRequest(
       (req, res) => {
-        res.writeHead(200, 'OK', {
+        res.set({
           'content-disposition': 'attachment',
           'content-type': 'application/json',
         })