Fix namespace in AssociationSet::build

Turns out I missed a case. Association namespaces are all checked at runtime since there's no `Batman.currentApp` when the association is defined. 

I missed that in `AssociationSet::build` and the tests pass because all the associations had explicit namespaces. At the same time, my local monkey patch didn't use options.namespace, only Batman.currentApp.

Anyways, here's a test and a fix!