Fix proxy tests

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #229 into master will increase coverage by 1.59%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   87.85%   89.44%   +1.59%     
==========================================
  Files          99       99              
  Lines        4832     4832              
==========================================
+ Hits         4245     4322      +77     
+ Misses        587      510      -77

Impacted Files	Coverage Δ
src/WireMock.Net/ResponseBuilders/Response.cs	95.73% <0%> (+1.16%)	⬆️
src/WireMock.Net/Server/FluentMockServer.Admin.cs	85.8% <0%> (+9.25%)	⬆️
...et/ResponseProviders/ProxyAsyncResponseProvider.cs	100% <0%> (+27.27%)	⬆️
src/WireMock.Net/Util/DictionaryExtensions.cs	100% <0%> (+100%)	⬆️

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