"Invalid Date" object triggers a show-stopping error

I was importing data from mysql to solr, and if I happened to have a date in my mysql table such as `0000-00-00 00:00:00`, then the `Date` object would become an "Invalid Date" and triggers a fatal error when trying to call `Date#toISOString()`.

This branch will check any `Date` object, replacing an "Invalid Date" with `null` and proceeding as normal for all others. I've also added a new test file for "utils" and have added basic tests for both `dateISOify()` and `toISOString()`.