superfluous response.WriteHeader call

`http: superfluous response.WriteHeader call from github.com/felixge/httpsnoop.(*Metrics).CaptureMetrics.func1.1 (capture_metrics.go:53)`

I sometimes get this error in my logs when serving static files with `http.ServeContent`.

Not sure how this happens if the wrapper is working correctly but the logs don't lie. Maybe `ServeContent` is somehow getting direct access to the underlying writer?