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