TAXIICollectionSource custom object handling fix

[emmanvg]

Resolves the unregistered custom object problem when a TAXII Server returns objects not registered on your local instance.

closes #459

[codecov-commenter]

Codecov Report

Merging #460 into master will decrease coverage by 8.99%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
- Coverage   98.24%   89.24%   -9.00%     
==========================================
  Files         142      142              
  Lines       15748    15748              
==========================================
- Hits        15471    14054    -1417     
- Misses        277     1694    +1417     

Impacted Files	Coverage Δ
stix2/datastore/taxii.py	69.74% <100.00%> (-18.49%)	⬇️
stix2/version.py	0.00% <0.00%> (-100.00%)	⬇️
stix2/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
stix2/v20/base.py	0.00% <0.00%> (-100.00%)	⬇️
stix2/v21/base.py	0.00% <0.00%> (-100.00%)	⬇️
stix2/v20/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
stix2/v21/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
stix2/v20/observables.py	21.71% <0.00%> (-78.29%)	⬇️
stix2/v21/observables.py	34.69% <0.00%> (-62.25%)	⬇️
stix2/v20/sdo.py	37.93% <0.00%> (-62.07%)	⬇️
... and 38 more

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