fix: Fix django-mongoengine support

[last-partizan]

@tfranzel Hi, this refactors one particular bugfix: 21068e0

I put this code into _map_model_field, becouse that fix has broken my previous fix: overriding _map_model_field to support django-mongoengine field types (MongoEngine/django-mongoengine@2ef9529).

This way it should work fine for both cases.

Refs #200

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.02 ⚠️

Comparison is base (4387fed) 98.62% compared to head (f585c11) 98.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #974      +/-   ##
==========================================
- Coverage   98.62%   98.61%   -0.02%     
==========================================
  Files          68       68              
  Lines        8245     8245              
==========================================
- Hits         8132     8131       -1     
- Misses        113      114       +1     

Impacted Files	Coverage Δ
drf_spectacular/openapi.py	97.33% <66.66%> (-0.11%)	⬇️

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