Correct data type in ObjectWithCreatedBy

Apparently this was a Timestamp before?

As its been working fine for months, it apparently doesn't actually matter, but it should be corrected anyways. There's also now a test to make sure that the property is of the correct type.
This could be:
```perl
my @property_objects = Genome::HasCreatedBy->__meta__->properties(property_name => 'created_by');
```
Or assert to UR there is only one:
```perl
my $property_object = Genome::HasCreatedBy->__meta__->property(property_name => 'created_by');
```