Case insensitive headers fix

[p1c2u]

Fixes #234

[codecov]

Codecov Report

Merging #236 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #236   +/-   ##
=======================================
  Coverage   96.79%   96.79%           
=======================================
  Files         123      123           
  Lines        2403     2403           
=======================================
  Hits         2326     2326           
  Misses         77       77           

Impacted Files	Coverage Δ
openapi_core/contrib/django/requests.py	100.00% <ø> (ø)
openapi_core/validation/request/datatypes.py	100.00% <100.00%> (ø)

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