code: public String asString() {
          String s = fromStringOrReader();
          if (s != null) return s;
          ByteBuffer bb = getByteBuffer();
          if (bb != null) return stringFrom(bb);
          InputStream is = fromStreamOrUrl();
          if (is != null) {
              Reader r = readerFrom(is);
              return stringFrom(r);
          }
          throw resourceUnaccessible();
      }
docs: Returns the resource as a string, converting it if necessary.
  <p>
  See the docs of {@linkplain RResult} for notes on data conversion.
  @return string