Fix MimePartMatcher and add more tests

[StefH]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 77.27273% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.19%. Comparing base (4b602dd) to head (1072527).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/WireMock.Net.Shared/Matchers/MatchResult.cs	63.26%	18 Missing ⚠️
.../WireMock.Net.Minimal/Matchers/CompositeMatcher.cs	72.97%	10 Missing ⚠️
...tchers/Request/RequestMessageHttpVersionMatcher.cs	20.00%	4 Missing ⚠️
...Matchers/Request/RequestMessageMultiPartMatcher.cs	63.63%	4 Missing ⚠️
.../Matchers/Request/RequestMessageClientIPMatcher.cs	71.42%	2 Missing ⚠️
...al/Matchers/Request/RequestMessageHeaderMatcher.cs	75.00%	2 Missing ⚠️
...nimal/Matchers/Request/RequestMessageUrlMatcher.cs	50.00%	2 Missing ⚠️
...d/Matchers/Helpers/BodyDataMatchScoreCalculator.cs	90.00%	2 Missing ⚠️
...eMock.Net.Minimal/Logging/WireMockConsoleLogger.cs	0.00%	1 Missing ⚠️
...Net.Minimal/Matchers/Request/RequestMatchResult.cs	92.30%	1 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1389      +/-   ##
==========================================
- Coverage   34.78%   34.19%   -0.59%     
==========================================
  Files         154      155       +1     
  Lines       37039    36363     -676     
==========================================
- Hits        12883    12435     -448     
+ Misses      23829    23643     -186     
+ Partials      327      285      -42     

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