add support for Change API's "Set Commit Message" endpoint

[srabraham]

Thanks for your excellent Gerrit API client! I wish Google offered something like this.

[codecov-io]

Codecov Report

Merging #80 into master will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   23.64%   23.56%   -0.08%     
==========================================
  Files          22       22              
  Lines        1844     1850       +6     
==========================================
  Hits          436      436              
- Misses       1342     1348       +6     
  Partials       66       66              

Impacted Files	Coverage Δ
changes.go	14.61% <0.00%> (-0.54%)	⬇️

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[srabraham]

I fixed the lint issue

[andygrunwald]

Thanks Sean, for taking the time and the PR!
Would you mind to add a basic unit test here as well?
Then we are ready to go and merge.

[srabraham]

Alright, added a couple of tests

[andygrunwald]

Thanks @srabraham!