Added support for multi-part body content

[mtabacman]

Fixes #43

[codecov]

Codecov Report

Merging #44 (6d8f576) into release-candidate (5bff032) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##           release-candidate      #44      +/-   ##
=====================================================
+ Coverage              99.60%   99.62%   +0.01%     
=====================================================
  Files                     50       53       +3     
  Lines                   2790     2924     +134     
=====================================================
+ Hits                    2779     2913     +134     
  Misses                    11       11              

Impacted Files	Coverage Δ
...ice-Discovery/FixedServiceDiscoveryClient.class.st	100.00% <ø> (ø)
...minal-Model-Tests/AddMultiPartCommandTest.class.st	100.00% <100.00%> (ø)
.../Superluminal-Model-Tests/HttpRequestTest.class.st	100.00% <100.00%> (ø)
...ce/Superluminal-Model/AddMultiPartCommand.class.st	100.00% <100.00%> (ø)
...Superluminal-Model/HttpRequestBodyBuilder.class.st	100.00% <100.00%> (ø)
...e/Superluminal-Model/MultiPartBodyBuilder.class.st	100.00% <100.00%> (ø)
...very-Tests/NullServiceDiscoveryClientTest.class.st	100.00% <100.00%> (ø)

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