Fix existing tests and add new tests. Fixes #363

[khdesai]

No description provided.

[codecov-io]

Codecov Report

Merging #376 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   98.18%   98.17%   -0.01%     
==========================================
  Files         124      124              
  Lines       14238    14278      +40     
==========================================
+ Hits        13980    14018      +38     
- Misses        258      260       +2

Impacted Files	Coverage Δ
stix2/test/v20/test_datastore_memory.py	100% <ø> (ø)	⬆️
stix2/test/v20/test_core.py	100% <ø> (ø)	⬆️
stix2/test/v21/test_core.py	100% <ø> (ø)	⬆️
stix2/test/v21/test_properties.py	100% <ø> (ø)	⬆️
stix2/test/v21/test_datastore_memory.py	100% <ø> (ø)	⬆️
stix2/test/v20/test_properties.py	100% <ø> (ø)	⬆️
stix2/test/v21/test_datastore_filesystem.py	99.22% <100%> (ø)	⬆️
stix2/test/v20/test_datastore_filesystem.py	99.2% <100%> (ø)	⬆️
stix2/exceptions.py	100% <100%> (ø)	⬆️
stix2/core.py	98.41% <100%> (-1.59%)	⬇️
... and 2 more

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