code: public static void addIfNotPresent(final HeaderGroup headers, final WarcHeader.Name name, final String value) {
  		if (! headers.containsHeader(name.value)) headers.addHeader(new WarcHeader(name, value));
  	}
docs: Adds the given header, if not present (otherwise does nothing).
  
  @param headers the headers where to add the new one.
  @param name the name of the header to add.
  @param value the value of the header to add.