LokiTarget - Expect100Continue for HttpClient

[snakefoot]

.NET Framework has Expect100Continue = true by default, so it is mostly an optimization for legacy platform.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.10%. Comparing base (b1c346b) to head (4ad01dc).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/NLog.Loki/LokiTarget.cs	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   76.42%   76.10%   -0.32%     
==========================================
  Files          12       12              
  Lines         403      406       +3     
  Branches       57       58       +1     
==========================================
+ Hits          308      309       +1     
- Misses         69       70       +1     
- Partials       26       27       +1     

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