api: dataset location is not actually assigned to ds_loc

... *ahem* ... rather embarassing this one. ds_loc is not actually
assigned and so if one tries to run a prediction, one will get an 500.

I really need to get the most basic of basic sanity checks working in
the test suite.
Merge pull request #90 from rjw57/fix-stupid-api-mistake

api: dataset location is not actually assigned to ds_loc