fix missing BOM in Producer metadata

I realized that my previous patch was not working as intended
 because it was missing the big endian BOM. It would get written
 to PDF without the BOM, which is contra the PDF standard.

Please note that almost all UTF-16BE strings actually get successfully
 converted to PDFDocEncoding by TextStringObject.writeToStream()
 So just because we write UTF-16BE doesn't mean the encoding will end
 up in the PDF.
Merge pull request #45 from chrishiestand/utf-16be-fix

fix missing BOM in Producer metadata