Proxy : also save multipart as string in mapping file

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #264 into master will increase coverage by 0.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   89.78%   90.22%   +0.44%     
==========================================
  Files         103      104       +1     
  Lines        5147     5380     +233     
==========================================
+ Hits         4621     4854     +233     
  Misses        526      526

Impacted Files	Coverage Δ
src/WireMock.Net/Server/FluentMockServer.Admin.cs	86.56% <100%> (+0.11%)	⬆️
src/WireMock.Net/Serialization/MatcherMapper.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Matchers/ExactObjectMatcher.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/ResponseMessageBuilder.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Util/BodyParser.cs	97.54% <100%> (+0.24%)	⬆️
src/WireMock.Net/Util/BytesEncodingUtils.cs	100% <100%> (ø)

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