if !ok {
		return io.CopyBuffer(writerOnly{w}, src, buf)
	}

	// Copy the first sniffLen bytes before switching to ReadFrom.
	// This ensures we don't start writing the response before the
	// source is available (see golang.org/issue/5660) and provides
	// enough bytes to perform Content-Type sniffing when required.
	if !w.cw.wroteHeader {
		n0, err := io.CopyBuffer(writerOnly{w}, io.LimitReader(src, sniffLen), buf)