flush data still available in the decoder when no input (#123)

… read

Unlike zlib, miniz_oxide consumes data into internal state
even if there is not enough space in the output buffer.
Next time poll_fill_buf() is called we should try
to decode from internal state even if no new compressed data
was read.

This change is a port of fix
<Nullus157#123>
(commit Nullus157@22ed0ac)
from `futures` to `tokio`.