Fix the flushing out of raw buffers on encoding conversions

https://bugzilla.gnome.org/show_bug.cgi?id=692915

the new set of converting functions tried to limit the encoding
conversion of the raw buffer to the consumption one to work in
a more progressive fashion. Unfortunately this was bad for
performances and led to errors on progressive parsing when
a very large chunk was close to the end of the document. Fix
the new internal function and switch back to the old way of
converting. Fix another bug in the process.