Fix ExactMatcher and JsonMatcher not working for ISO dates as string

[StefH]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 36.61%. Comparing base (6e2a4d7) to head (712e786).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...ock.Net.Minimal/Serialization/MappingSerializer.cs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1443      +/-   ##
==========================================
+ Coverage   36.59%   36.61%   +0.02%     
==========================================
  Files         157      157              
  Lines       38380    38381       +1     
==========================================
+ Hits        14044    14054      +10     
+ Misses      23804    23795       -9     
  Partials      532      532              

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