code: public HttpHeaders set(String name, String value) {
          final String headerKey = name.toLowerCase(Locale.ROOT);
          if (value == null) {
              headers.remove(headerKey);
          } else {
              headers.put(headerKey, new HttpHeader(name, value));
          }
          return this;
      }
docs: Set a header.
  
  if header with same name already exists then the value will be overwritten.
  if value is null and header with provided name already exists then it will be removed.
  
  @param name the name
  @param value the value
  @return this HttpHeaders