fix error on read-only serializer

[treussart]

see #404

[codecov]

Codecov Report

Merging #405 (d71b5f8) into master (f58bbb0) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   98.55%   98.51%   -0.04%     
==========================================
  Files          55       55              
  Lines        5384     5387       +3     
==========================================
+ Hits         5306     5307       +1     
- Misses         78       80       +2     

Impacted Files	Coverage Δ
drf_spectacular/openapi.py	96.39% <50.00%> (-0.27%)	⬇️

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[tfranzel]

awesome! thx. will likely add a test tomorrow.