compare decoded go struct for request body

[willnorris]

The use of cmp.AllowUnexported stuck out to me as code smell when reviewing #1875, but since the weird comparison was already there, I decided to leave it alone in that PR.

This was a very weird test method, since it doesn't follow the pattern of the rest of go-github for comparing POST request bodies, and as a result makes things way more complicated than they need to be.

This eliminates the GetReadWriter and GetReadCloser funcs. There was discussion in #1238 about these potentially being general purpose funcs, but they never ended up being used beyond this one method, nor should they ever be necessary I don't think. Even though these were exported funcs, because they were only defined in the test package, this shouldn't be a breaking change.

[codecov]

Codecov Report

Merging #1876 (17fb4ff) into master (51e7595) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1876   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files         105      105           
  Lines        6783     6783           
=======================================
  Hits         6624     6624           
  Misses         86       86           
  Partials       73       73           

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[gmlewis]

Thank you, @willnorris !
LGTM.