Fix for NullReferenceException when GetService<IHttpClientFactory>() fails

[amazur31]

Fix for this #98

It's 2:20 AM I was stuck on this error for too long... To whoever didn't fix it the first time - fuck you. To the creator of AspNetCore.Proxy - thank you.

[twitchax]

@amazur31, can you also add a test for this? Basically, want to make sure this doesn't get regressed.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.97%. Comparing base (b3f70af) to head (e9393fd). Report is 1 commits behind head on master.

Files	Patch %	Lines
src/Core/Extensions/Http.cs	66.66%	0 Missing and 1 partial :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   99.85%   98.97%   -0.88%     
==========================================
  Files          12       12              
  Lines         686      686              
  Branches       84       86       +2     
==========================================
- Hits          685      679       -6     
  Misses          1        1              
- Partials        0        6       +6     

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