Use `.toISOString()` in DateAssertion messages

The DateAssertions messages usually show a stringified version of the date(s). It'd be much easier to read the messages if we show the dates using the ISO format, which is shorter and a well-known standard as well.

#### Current format:
```
Fri Jul 01 2022 17:19:16 GMT-0500 (Ecuador Time)
```

#### Expected ISO format:
```
2022-07-01T22:19:16.383Z
```