bug#2878: bindat-pack returns unibyte string for emacs 22 but multibyte string for emacs 23

Hi,

This piece of code returned nil on emacs 22 and t on emacs 23.
(let* ((spec `((:method-id     vec 4)))
       (data  `((:ref-type  . ,(vector #x00 #x00 #x00 #x00 #x00 #x00 #x00 #x39))
		(:method-id . ,(vector #x09 #x90 #xec #xd8))))
       (packed (bindat-pack spec data)))
  (multibyte-string-p packed))

And because of it being multibyte, when i want to send it over the
wire to a sub process in a binary data format, it gets transformed and
the sub process does not get a byte-by-byte equivalent of the
concatenation of the 2 vectors above. I have tried using
string-as-unibyte and the effect is still the same.

-- 
Cheers,
Phuah Yee Keat