Fix issue saving S4 objects as RDS

[jkanche]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.54930% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.34%. Comparing base (8e014c6) to head (c45ec20).

Files with missing lines	Patch %	Lines
src/rds2py/save_compressed_list.py	68.18%	6 Missing and 1 partial ⚠️
src/rds2py/read_sce.py	60.00%	1 Missing and 1 partial ⚠️
src/rds2py/save_atomic.py	85.71%	1 Missing and 1 partial ⚠️
src/rds2py/save_matrix.py	95.23%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #77       +/-   ##
===========================================
+ Coverage   83.54%   94.34%   +10.79%     
===========================================
  Files          29       29               
  Lines         924     1044      +120     
  Branches      137      160       +23     
===========================================
+ Hits          772      985      +213     
+ Misses         74       23       -51     
+ Partials       78       36       -42     

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