Validate outputs of ic_write()

Plenty of issue at https://icalendar.org/validator.html.

In one example:

```
Lines not delimited by CRLF sequence near line # 1Reference: RFC 5545 3.1. Content Lines
Missing DTSTAMP property near line # 6Reference: RFC 5545 3.6.1. Event Component
UID value is not unique, duplicate found near line # 6
```