binary strings shouldn't be converted to UTF-16
update FileWriter to use XRef Streams in files that already use them

* several PDF readers seem to cope fine when opening a PDF that was
  originally saved with an XRef stream and has an incremental update
  that uses a traditional xref table. Adobe is not one of them, it
  refuses to open such files.
* The only solution is to detect which xref style is in use and then
  write any incremental updates in the appropriate style