code: private String takeString() {
          String s = _string.toString();
          // trim trailing whitespace.
          if (s.length() > _length)
              s = s.substring(0, _length);
          _string.reset();
          _length = -1;
          return s;
      }
docs: /*
  Mime Field strings are treated as UTF-8 as per https://tools.ietf.org/html/rfc7578#section-5.1