Mandatory properties are neither enforced nor set to a default

[danielschenk]

According to RFC 5545, every VCAL needs a PRODID and VERSION. If those are not explicitly added, Calendar.to_ical() produces non-conforming output because they are missing.

It could be a deliberate design choice to leave this open. But I wonder, how often do people want to generate invalid output, and shouldn't the library enforce a valid calendar object somehow?

Just some suggestions here.

VERSION

As this library claims to be implementing RFC 5545, I think this should just be 2.0 by default. If you add it again with a different value, that's fine, it will be replaced.

PRODID

What about setting this to something like -//python.collective.icalendar//NONSGML python.collective.icalendar.app.event//EN? (I'm not sure how exactly these strings should be constructed, I just wrote something based on a test case I saw in this repo). Same here, if you add it again with a different value, it will be replaced.

Both of above improvements are fully backwards-compatible, AFAIK. Please let me know your thoughts.

[niccokunzmann]

I would think that is is nice to add a few default properties that make the calendar valid. If you like to create a Pull Request, that would be welcome!

[danielschenk]

I'll gladly do! I will dive into the code as soon as I have time for it.