Adding headers in GithubException is a breaking change

[s-t-e-v-e-n-k]

Since exporting headers in GithubException might require passing them in
a constructor for client code, shift it as a breaking change so users
watch out for it.

[codecov-commenter]

Codecov Report

Merging #1931 (fd3dfb2) into master (8aebd94) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1931   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files         108      108           
  Lines       11064    11064           
=======================================
  Hits        10937    10937           
  Misses        127      127           

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