[ADD] OSX check to FileSystemProvider

[geoperez]

Closes #423

[codecov[bot]]

Codecov Report

Merging #424 into master will decrease coverage by 2.61%. The diff coverage is 69.48%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
- Coverage   59.94%   57.32%   -2.62%     
==========================================
  Files         164      168       +4     
  Lines        6745     6828      +83     
  Branches     1211     1278      +67     
==========================================
- Hits         4043     3914     -129     
- Misses       2364     2443      +79     
- Partials      338      471     +133

Impacted Files	Coverage Δ
src/EmbedIO/WebServerBase`1.cs | 75.65% <0%> (-4.35%)	:arrow_down:
src/EmbedIO/Security/BannedInfo.cs	33.33% <0%> (-66.67%)	:arrow_down:
src/EmbedIO/WebModuleBase.cs	63.63% <0%> (-2.28%)	:arrow_down:
src/EmbedIO/ExceptionHandler.cs	58.49% <42.85%> (-26.93%)	:arrow_down:
...c/EmbedIO/WebModuleContainerExtensions-Security.cs	50% <50%> (ø)	:arrow_up:
src/EmbedIO/Security/IPBanningRequestsCriterion.cs	61.53% <61.53%> (ø)
src/EmbedIO/Security/IPBanningExecutor.cs	69.23% <69.23%> (ø)
src/EmbedIO/Security/IPBanningConfiguration.cs	69.35% <69.35%> (ø)
src/EmbedIO/Security/IPBanningRegexCriterion.cs	72.3% <72.3%> (ø)
src/EmbedIO/Security/IPBanningModuleExtensions.cs	57.14% <75%> (+7.14%)	:arrow_up:
... and 49 more

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[geoperez]

I modified the IsImmutable logic.