Fix flask decorator function return other types

[p1c2u]

Fixes #488

[codecov]

Codecov Report

Merging #489 (a50ff1d) into master (221d212) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #489   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         105      105           
  Lines        2967     2969    +2     
  Branches      255      255           
=======================================
+ Hits         2736     2738    +2     
  Misses        189      189           
  Partials       42       42           

Impacted Files	Coverage Δ
openapi_core/contrib/flask/decorators.py	100.00% <100.00%> (ø)

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