code: private List<String> getHeaderValues(String name) {
          List<String> values = headers.get(name);
          if (values == null) {
              values = new ArrayList<String>();
              headers.put(name, values);
          }
          return values;
      }
docs: Convenience method to get a header value list, initialize it and put it in the headers map
  if it does not exist
  
  @param name header name
  @return list containing values for this header