Respect FLAG_FIN & FLAG_RST in TYPE_WINDOW_UPDATE frames

[vegas503]

What was wrong?

Issue #931

How was it fixed?

Added handling of FIN & RST flags in WINDOW_UPDATE frames.

To-Do

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

[paschal533]

Hi @vegas503, The fix looks correct overall. Would you be able to add some unit tests for this? Specifically testing WINDOW_UPDATE frames with FLAG_FIN and FLAG_RST.
One question: have you tested this with any real yamux implementations to make sure the behavior matches expectations?
Overall this looks like a solid fix for a real spec compliance issue. Once we address the testing and potential refactoring, I think this will be ready to merge. Well done @vegas503 🙌