[FEATURE] Enable TimeSpan with null end

[calien666]

This commit allows creating TimeSpan Object only with DTSTART set. As a starting time without ending is allowed in iCal definitions, this package should respect the ability having no end time set, too.

Fixes #561

[markuspoerschke]

Moin Markus (@calien666),

thanks for your PR.

I have one question to this change:

What is the difference between a TimeSpan object with no end and a SingleDay object? Do you need to have an event without end-time in the middle of the week? If you want to have an event that just points to a day, then you can do it using the SingleDay.

Can you outline the use case here? I would be interested into the look in the calendar app as well. Would be nice to see a screenshot.

[calien666]

Moin Markus (@markuspoerschke), The difference between a TimeSpan without end and a SingleDay Event is, the event has a starting time, but no ending, while a SingleDay event is whole day located.

The most affectible use case ist, the importing calendar decides, how long the event will be lasting according to the default timespan set up by the calendar default settings. I'll take a look on an example.

Possibly there could be a separate class added, which is called TimeStart or something instead of nulling TimeSpan end time. But I'm not sure, if this is a possible use or not. Let me check, how calendar applications handle these events and then I can make a better descision.

[markuspoerschke]

Understand. Makes sense in my opinion. Also, I would keep the way you implemented it.

Can you please adjust the following in your pull-request?

1. Add an entry to the changelog file
2. Add at least one test that is covering the new feature
3. Add instructions to the documentation how to use the open end time span

[calien666]

Hey @markuspoerschke, sorry for reacting that late. I hope, I will finish it this week.