Make X509CertificatePassword optional

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #732 (7f57d0d) into master (344f5c8) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
- Coverage   75.72%   75.68%   -0.04%     
==========================================
  Files         175      175              
  Lines        6340     6343       +3     
  Branches      646      647       +1     
==========================================
  Hits         4801     4801              
- Misses       1346     1349       +3     
  Partials      193      193              

Impacted Files	Coverage Δ
...WireMock.Net/HttpsCertificate/CertificateLoader.cs	0.00% <0.00%> (ø)
src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs	74.07% <0.00%> (ø)
...reMock.Net/Settings/WireMockCertificateSettings.cs	0.00% <0.00%> (ø)

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