Allow "value" in DatasetSpec

[rly]

Motivation

Fix #1142

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.83%. Comparing base (eb67626) to head (7367733).
Report is 27 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1143   +/-   ##
=======================================
  Coverage   88.83%   88.83%           
=======================================
  Files          45       45           
  Lines        9778     9784    +6     
  Branches     2777     2779    +2     
=======================================
+ Hits         8686     8692    +6     
  Misses        776      776           
  Partials      316      316           

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