Fix ContentType with parameters in Proxy Mode

[StefH]

• remove MimeKitLite and use MediaTypeHeaderValue

[codecov]

Codecov Report

Merging #338 into master will increase coverage by 0.02%.
The diff coverage is 95.4%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
+ Coverage   80.14%   80.16%   +0.02%     
==========================================
  Files         111      111              
  Lines        4452     4442      -10     
==========================================
- Hits         3568     3561       -7     
+ Misses        884      881       -3

Impacted Files	Coverage Δ
src/WireMock.Net/Http/HttpResponseMessageHelper.cs	85.71% <ø> (ø)	⬆️
src/WireMock.Net/Http/HttpRequestMessageHelper.cs	86.84% <100%> (ø)	⬆️
src/WireMock.Net/Http/StringContentHelper.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Http/HttpClientHelper.cs	82.14% <100%> (ø)	⬆️
src/WireMock.Net/Util/BodyParser.cs	93.75% <95.06%> (+2.84%)	⬆️

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