For internal methods, don't pass istream as an argument since we use this.istream. Added method seek(offset).
For internal methods, don't pass istream as an argument since we use this.istream.
Added method seek(offset).

wentaoshang/NDN-on-Node,wentaoshang/NDN-on-Node,wentaoshang/NDN.JS,wentaoshang/NDN.JS