Correctly support DateTime pattern as string in ExactMatcher

[StefH]

#383

[codecov]

Codecov Report

Merging #391 into master will decrease coverage by 0.07%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage      80%   79.93%   -0.08%     
==========================================
  Files         121      121              
  Lines        4782     4789       +7     
==========================================
+ Hits         3826     3828       +2     
- Misses        956      961       +5

Impacted Files	Coverage Δ
src/WireMock.Net/Util/BodyParser.cs	96.96% <100%> (-0.04%)	⬇️
src/WireMock.Net/ResponseBuilders/Response.cs	96.96% <100%> (ø)	⬆️
src/WireMock.Net/Server/FluentMockServer.Admin.cs	61.01% <16.66%> (-0.21%)	⬇️
src/WireMock.Net/Util/JsonUtils.cs	96.84% <50%> (-3.16%)	⬇️

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