I think that You are missing one thing here. GET method does not contain any payload, therefore it can't have content type header set.
If you want to override response mime (like in first example) you shouldn't use header because it has different purpose. Unfortunately I can't see a way to override response's mime type in agular's HTTP class. I think there isn't one. So maybe you can use HttpRequest class in your service for now and file an issue report.