Fix bug when adding custom object to FileSystemSink if the object type hasn't been registered

[clenk]

Related: #429.

[codecov-commenter]

Codecov Report

Merging #431 into master will increase coverage by 0.00%.
The diff coverage is 96.90%.

@@           Coverage Diff           @@
##           master     #431   +/-   ##
=======================================
  Coverage   98.37%   98.37%           
=======================================
  Files         130      131    +1     
  Lines       14886    14936   +50     
=======================================
+ Hits        14644    14694   +50     
  Misses        242      242           

Impacted Files	Coverage Δ
stix2/__init__.py	100.00% <ø> (ø)
stix2/utils.py	96.66% <ø> (+0.74%)	⬆️
stix2/serialization.py	94.73% <94.73%> (ø)
stix2/base.py	96.85% <100.00%> (+0.09%)	⬆️
stix2/datastore/filesystem.py	94.42% <100.00%> (+0.11%)	⬆️
stix2/test/v20/test_datastore_filesystem.py	99.21% <100.00%> (+0.01%)	⬆️
stix2/test/v20/test_utils.py	100.00% <100.00%> (ø)
stix2/test/v21/test_datastore_filesystem.py	99.23% <100.00%> (+0.01%)	⬆️
stix2/test/v21/test_utils.py	100.00% <100.00%> (ø)
stix2/v20/bundle.py	100.00% <0.00%> (ø)
... and 2 more

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[emmanvg]

General comment for this PR is that serialization logic being removed from _STIXBase and being standalone it can follow a similar path to the versioning.py or parsing.py modules. Perhaps we could move the serialize, STIXJSONEncoder and the STIXJSONIncludeOptionalDefaultsEncoder to their own separate module.

This would also remove the them from the __all__ override under base.py

[chisholm]

While you're at it... what about the find_property_index() stuff from utils? It's only used for pretty-serialization. Maybe it belongs in serialization.py too.

[emmanvg]

Agree @chisholm, will make the change

[clenk]

These changes look good to me!

[emmanvg]

LGTM with overall changes.