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?

从 capture_metrics.go:53 开始，跟踪 httpsnoop 在对静态文件执行 http.ServeContent 期间如何包装 response writer。使用一个最小 handler 重现报告中的日志，然后将 wrapper 的 WriteHeader 调用与 ServeContent 的行为进行比较。当原因得到确定，且受影响情况下不再出现 superfluous-call log 时，即视为完成。

由索引模型根据 Issue 内容生成。