Flush lame encoder.

I used flush_nogap because I think that we don't generally want id3 tags to be written in the stream. I did not directly commit, because this change is so obvious that I fear there might have been some good reason for not doing it earlier. So waiting for @toots or @dbaelde comments (and tests:)).

I have tested reencoding of an mp3 file and I do get a few more sample at the end.