Fixes oauth/access_token Interpreter

[juanpedromoreno]

This PR fixes the oauth/access_token.

Essentially, the POST information needs to be passed by URL parameters, instead of in the body, as you can see in this section.

[codecov]

Codecov Report

Merging #509 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
+ Coverage   90.37%   90.45%   +0.07%     
==========================================
  Files          24       24              
  Lines         613      618       +5     
  Branches        3        1       -2     
==========================================
+ Hits          554      559       +5     
  Misses         59       59              

Impacted Files	Coverage Δ
github4s/src/main/scala/github4s/Encoders.scala	96.42% <ø> (-0.13%)	⬇️
...ub4s/src/main/scala/github4s/http/HttpClient.scala	87.50% <100.00%> (+0.54%)	⬆️
.../scala/github4s/interpreters/AuthInterpreter.scala	100.00% <100.00%> (ø)

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