dstm: setting values to .ts.attributes needs fix

Overview

As reported by @karakcia and @ole.andreas.ramsdal ,

Given the case:

1. minimal dstm model, with some ts.attribute=shyft://something
2. evaluate the model (to pull in first set of values)
3. then post a set_request on part of the ts.attribute (new changed values)

Expected(wanted) outcome:

Data stored to dtss, and notification is flying around

Observed outcome:

web response shows [] (a bit strange considering the code in question)

DONE

Check test-cases, reconstruct/extend cases to cover the specific steps.

Add extra handling for the setter, to direct it to the dtss