Fixed logic for IsRestrictedResponseHeader

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #228 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   87.81%   87.85%   +0.04%     
==========================================
  Files          98       99       +1     
  Lines        4816     4832      +16     
==========================================
+ Hits         4229     4245      +16     
  Misses        587      587

Impacted Files	Coverage Δ
...rc/WireMock.Net/Owin/Mappers/OwinResponseMapper.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Http/HttpKnownHeaderNames.cs	100% <100%> (ø)

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