Fix missed content headers

[volodymyr-fed]

We need to pass the content headers even if the content is empty.

• fixed issue with passing content headers
• added test for checking the content header when the content is empty

[codecov]

Codecov Report

Merging #79 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #79   +/-   ##
=======================================
  Coverage   54.23%   54.23%           
=======================================
  Files          66       66           
  Lines        2410     2410           
  Branches      331      331           
=======================================
  Hits         1307     1307           
  Misses        964      964           
  Partials      139      139

Impacted Files	Coverage Δ
src/WireMock.Net/Http/HttpClientHelper.cs	76.56% <100%> (ø)	⬆️

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[StefH]

New version (1.0.2.13) will be uploaded to NuGet in a while.

[volodymyr-fed]

Thanks