SAR-55 Fix date format in get_default_formatter

The `datefmt` string was not being set correctly resulting in the
incorrect output:
```
1|2020-01-13 10:42:42,415.415Z|...
```
instead of:
```
1|2020-01-13T10:42:42.415Z|...
```

Tests now ensure that the output from the default configuration
matches the output from the standalone formatters.