code: func ContentCharset(charsets ...string) func(next http.Handler) http.Handler {
  	for i, c := range charsets {
  		charsets[i] = strings.ToLower(c)
  	}
  
  	return func(next http.Handler) http.Handler {
  		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  			if !contentEncoding(r.Header.Get("Content-Type"), charsets...) {
  				w.WriteHeader(http.StatusUnsupportedMediaType)
  				return
  			}
  
  			next.ServeHTTP(w, r)
  		})
  	}
  }
docs: // ContentCharset generates a handler that writes a 415 Unsupported Media Type response if none of the charsets match.
  // An empty charset will allow requests with no Content-Type header or no specified charset.