Can't access `self.service_url` in `ScienceBaseResource._get`

[nikmolnar]

The ._get method of ScienceBase resource needs to access self.service_url in order to correctly generate (or not) an ArcGIS server token:

server_admin = ArcGISSecureResource.generate_token(self.service_url, arcgis_user, arcgis_pass)

However, due to circular resource loading logic, the service_url getter method returns ._url as a default because trying to load ._service_url results in an attribute error.

Likewise, trying to use ._service_url directly in in the ScienceBaseResource._get method causes an AttributeError and resource loading fails.

This issue can be observed in 1589ea8, which contains a shim to work around it. Without the extra call to self.service_url, it will end up using ._url to try generate an ArcGIS server token.