Handle invalid JSON on error responses

[JSkimming]

Unauthorized responses state the Content-Type is application/json, but the body is not valid JSON.

Here is an example response.

HTTP/1.1 401 Unauthorized
Server: nginx
Date: Thu, 01 Feb 2018 07:58:39 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
X-Content-Type-Options: nosniff
Cache-Control: no-store
Pragma: no-cache
WWW-Authenticate: Bearer realm="Doorkeeper", error="invalid_token", error_description="The access token is invalid"
X-Request-Id: b05b4cc2-7684-49bb-b2cc-57c548d4a2cf
X-Runtime: 0.024554

0

[codecov]

Codecov Report

Merging #16 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #16   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines         567    571    +4     
  Branches       71     72    +1     
=====================================
+ Hits          567    571    +4

Impacted Files	Coverage Δ
src/Tesla.NET/Requests/HttpClientExtensions.cs	100% <100%> (ø)	⬆️

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