Add TokenExpiration to Response

[w-haibara]

GitHub has been updated to allow you to set the expiration date of your personal access token. And the expiration date is now displayed in the GitHub-Authentication-Token-Expiration header of the GItHub API.
https://github.blog/changelog/2021-07-26-expiration-options-for-personal-access-tokens/

This PR add TokenExpiration field to Response.

[gmlewis]

Thank you, @w-haibara !

In addition to the comments below, please add a new unit test for the new parseTokenExpiration function.

Also please make sure to run gofmt on the edited files so that their formatting is standardized.

See CONTRIBUTING.md in the top-level of this repo for more information.

[codecov]

Codecov Report

Merging #2031 (d659626) into master (d8a66d0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2031   +/-   ##
=======================================
  Coverage   97.86%   97.87%           
=======================================
  Files         107      107           
  Lines        6901     6907    +6     
=======================================
+ Hits         6754     6760    +6     
  Misses         81       81           
  Partials       66       66           

Impacted Files	Coverage Δ
github/github.go	97.15% <100.00%> (+0.04%)	⬆️

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[w-haibara]

Hi, @gmlewis ! Thanks for the thoughtful review.
I've fixed the areas you commented on and added a test. I apologize again for the unformatted commits. I've fixed that as well.

[gmlewis]

Thank you, @w-haibara !

[gmlewis]

Thank you, @w-haibara !
LGTM.

Awaiting second LGTM before merging.

[w-haibara]

Thank you, @gmlewis.
This is almost the first OSS contributor for me. Your review has helped me a lot!

[gmlewis]

Thank you, @gmlewis.
This is almost the first OSS contributor for me. Your review has helped me a lot!

Thank you, for your contributions, @w-haibara ! Your help is greatly appreciated!

[Parker77]

LGTM!