code: func (w *gzipResponseWriter) WriteHeader(code int) {
  	w.Header().Del("Content-Length")
  	w.Header().Set("Content-Encoding", "gzip")
  	w.Header().Add("Vary", "Accept-Encoding")
  	w.ResponseWriter.WriteHeader(code)
  	w.statusCodeWritten = true
  }
docs: // WriteHeader wraps the underlying WriteHeader method to prevent
  // problems with conflicting headers from proxied backends. For
  // example, a backend system that calculates Content-Length would
  // be wrong because it doesn't know it's being gzipped.