Use dotnet default development certificate for .NET Core 2.x

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #381 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #381   +/-   ##
=======================================
  Coverage   80.32%   80.32%           
=======================================
  Files         117      117           
  Lines        4701     4701           
=======================================
  Hits         3776     3776           
  Misses        925      925

Impacted Files	Coverage Δ
...ck.Net/HttpsCertificate/PublicCertificateHelper.cs	0% <ø> (ø)	⬆️
src/WireMock.Net/Owin/AspNetCoreSelfHost.cs	89.58% <100%> (ø)	⬆️

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