ServeError should propertly set headers

[minigolf2000]

Successful responses are correctly being handled. But currently there's a bug where ServeError returns the response body without actually setting headers. As a result, Content-Length is not set, and Content-Type ends up defaulted to text/plain charset=utf-8.

The reason is because https://golang.org/pkg/net/http/#ResponseWriter

Changing the header map after a call to WriteHeader (or
Write) has no effect unless the modified headers are
trailers.

The correct order of operations, for both success and failure, is:

1. set headers with w.Header().set()
2. set response code through w.WriteHeader()
3. w.Write()

I noticed Vinyl has this bug too lol we should fix that at some point.