code: public String getHeader(HttpString headerName) {
          return (this.httpServerExchange.getRequestHeaders().get(headerName) == null) ? null : this.httpServerExchange.getRequestHeaders().get(headerName).element();
      }
docs: Retrieves a specific header value by its name
  
  @param headerName The name of the header to retrieve
  @return The value of the header or null if none found