Fix issue 309

[khdesai]

No description provided.

[codecov-io]

Codecov Report

Merging #316 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   98.15%   98.16%   +<.01%     
==========================================
  Files         123      123              
  Lines       13913    13919       +6     
==========================================
+ Hits        13657    13663       +6     
  Misses        256      256

Impacted Files	Coverage Δ
stix2/v21/observables.py	97.16% <100%> (+0.01%)	⬆️
stix2/test/v21/test_observed_data.py	100% <100%> (ø)	⬆️

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[clenk]

Thanks @khdesai - I rebased and merged it in.