Fix double 'BZBZ' magic string regression in bzip2 archives

Commit ce4bf4c5 introduced a regression where archives created with
plexus-archiver would only be opened by plexus-archiver and no other tool. In
similar fashion plexus-archiver was unable to extract proper archives because it
removed "BZ" magic header before passing the stream to commons-compress

Signed-off-by: Kristian Rosenvold <krosenvold@apache.org>