FastBootAppServer duplicates response headers

FastBootAppServer duplicates response headers which were set by middleware registered in beforeMiddleware hook.


I've found that it happens during fastboot-express-middleware execution, here:
https://github.com/ember-fastboot/ember-cli-fastboot/blob/master/packages/fastboot-express-middleware/src/index.js
lines: 44-46

From my perspective, the problem is that code doesn't check already existing response headers before adding new headers from `result.headers`.

Here, you can find a prepared by me demo of the issue:
https://github.com/yevhenii-pinkal/fastboot-duplicates-headers-issue/tree/master