CompressionStream.Flush: also flush innerStream

[lostmsu]

fixes #66

[oleg-st]

I briefly looked at DeflateStream/BrotliStream, inner Flush is done after Flush, but not after Dispose (with leaveOpen true). So it needs to be done in Flush/FlushAsync, not in FlushInternal/FlushInternalAsync.

[lostmsu]

@oleg-st I think not doing inner Flush in FlushInternal* might lead to mistakes in the future. Either these methods should be renamed or they should flush the inner stream.

[oleg-st]

We can rename FlushInternal to WriteNothing or WriteEmpty or something like this

[lostmsu]

Doesn't it actually commit the compressor block or something like that?

[oleg-st]

It depends on directive argument

[oleg-st]

Added PR #68 with you as co author

@lostmsu Could you check it?