Fix ShutdownOnDispose to work with isolated LogFactory

[snakefoot]

Fixing TODO. Resolves #442

[codecov-commenter]

Codecov Report

Merging #444 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #444   +/-   ##
=======================================
  Coverage   82.22%   82.22%           
=======================================
  Files          14       14           
  Lines        1198     1198           
  Branches      200      200           
=======================================
  Hits          985      985           
  Misses        141      141           
  Partials       72       72           

Impacted Files	Coverage Δ
...g.Extensions.Logging/Logging/NLogLoggerProvider.cs	95.23% <100.00%> (ø)

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[304NotModified]

Thanks!