Fix handling of modified recurrences with lower sequence number than their base event

[dstolpmann]

Hi,
the current implementation assumes that a modified recurrence has a higher sequence number than its base event. However, this is not always the case, as modifying the base event after creating a modified recurrence does only increase the sequence number of the base event.

This pull request fixes this behavior by also using the presents of the sequence ID field to determine if an event replaces another one. I am not sure if this behavior is according to the specification, but this topic seems to be highly controversial as stated in this article: https://icalevents.com/4437-correct-handling-of-uid-recurrence-id-sequence/

Best regards,
dstolpmann

[niccokunzmann]

Since you did the research, I am the more happy to merge this.

With the assumption, you are right. I did not know how to handle this.

Thanks for adding an example and tests!
I wonder: The default case should already be accounted for in other places, I believe. So, there is no test necessary, right?

[niccokunzmann]

Following the release process in the README file, this change can be documented in the changelog. Would you like to create a PR for it, mentioning Issue and to your liking your username in a kind of consistent format with that what was noted down in the changelog before?

[dstolpmann]

Sure! Should I also increase the version number to v0.1.20b?

[niccokunzmann]

@dstolpmann yes!