property self_link is always ''

# Description
For all created entities, the self_link is always `''`

## Reproduce the error
```PYTHON
...
point = Point((-115.81, 37.24))
location = fsc.Location(name="here", description="and there", location=point, encoding_type='application/geo+json')
service.create(location)
print('->' + location.self_link + '<-')
```
prints `-><-`