code: private static boolean isHeaderAllowingMultipleValues(final Buffer headerName) {
          final int size = headerName.getReadableBytes();
          if (size == 7) {
              return !isSubjectHeader(headerName);
          } else if (size == 5) {
              return !isAllowHeader(headerName);
          } else if (size == 4) {
              return !isDateHeader(headerName);
          } else if (size == 1) {
              return !isAllowEventsHeaderShort(headerName);
          } else if (size == 12) {
              return !isAllowEventsHeader(headerName);
          }
  
          return true;
      }
docs: Not all headers allow for multiple values on a single line. This is a
  basic check for validating whether or not that the header allows it or
  not. Note, for headers such as Contact, it depends!
  
  @param headerName
  @return