code: public HeaderField remove() {
      HeaderField removed = headerFields[tail];
      if (removed == null) {
        return null;
      }
      size -= removed.size();
      headerFields[tail++] = null;
      if (tail == headerFields.length) {
        tail = 0;
      }
      return removed;
    }
docs: Remove and return the oldest header field from the dynamic table.