fix discriminator not marshalled correctly when using polymorph fields

[KeyzaneX]

Fix issue #314

clear cache on resolved method to force discriminator to be re evaluated
Editing test to make it more useful

[codecov]

Codecov Report

Merging #359 (6d4a820) into master (e1ab7e3) will decrease coverage by 0.58%.
The diff coverage is 100.00%.

❗ Current head 6d4a820 differs from pull request most recent head c96a242. Consider uploading reports for the commit c96a242 to get more accurate results

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
- Coverage   96.86%   96.27%   -0.59%     
==========================================
  Files          20       20              
  Lines        2740     2741       +1     
==========================================
- Hits         2654     2639      -15     
- Misses         86      102      +16     

Impacted Files	Coverage Δ
flask_restx/fields.py	96.24% <100.00%> (-0.21%)	⬇️
flask_restx/schemas/__init__.py	80.32% <0.00%> (-3.28%)	⬇️
flask_restx/cors.py	78.04% <0.00%> (-2.44%)	⬇️
flask_restx/resource.py	96.15% <0.00%> (-1.93%)	⬇️
flask_restx/model.py	95.55% <0.00%> (-1.49%)	⬇️
flask_restx/swagger.py	95.48% <0.00%> (-1.26%)	⬇️
flask_restx/marshalling.py	97.32% <0.00%> (-0.90%)	⬇️
flask_restx/reqparse.py	97.87% <0.00%> (-0.86%)	⬇️
flask_restx/api.py	96.52% <0.00%> (-0.24%)	⬇️
... and 1 more

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[j5awry]

I'm seeing failures in py2 w/ cannot delete attribute. i also saw this on py3.5 (rerunning tests is a bit annoying with these runners). So something isn't quite setup correctly to work on py3.5, which we still support (and as a Canonical employee, I'm inclined to continue to support until Xenial ESM expires)

[KeyzaneX]

after few hours of reading code, It looks like we don't need to use "cached_property" , It works great with @Property instead.
tested on py2.7 and 3.5