flush data still available in the decoder when no input

[nl5887]

closes #123

[michalfita]

@nl5887 any chance this could be pushed up the priorites list, please? This fix works for us for some time and we'd really like to return to upstream releases.

Thanks in advance.

[Nemo157]

I would really like a testcase with this to make sure it does fix the issue and there won't be a regression. I'll probably take another try at writing one myself, but I remember failing at it once already.

[michalfita]

@nl5887 isn't recently very active on GH, hopefully he gets this ping.

[robjtede]

Since this PR has not allowed maintainer edits, I've rebased and opened #247.