Don't update the ratelimits if we got a response from a cache

[Jille]

The recommended cache, https://github.com/gregjones/httpcache, returns
an X-From-Cache: 1 header whenever it serves a response from the cache.

Cached responses might include (possibly stale) ratelimit headers, which
we would've already processed in the past.

[codecov]

Codecov Report

Merging #2273 (2b58fd7) into master (a842efc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2273   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files         115      115           
  Lines       10360    10363    +3     
=======================================
+ Hits        10136    10139    +3     
  Misses        156      156           
  Partials       68       68           

Impacted Files	Coverage Δ
github/github.go	97.64% <100.00%> (+0.01%)	⬆️

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[gmlewis]

Thank you, @Jille !
Can you please add a unit test that demonstrates this change in functionality?

Also, once you sign the CLA, we can move forward with this PR.

[Jille]

Thanks for the quick review :)

Added tests, rebased to master, signed the CLA.

[gmlewis]

Thank you, @Jille !
LGTM.

Awaiting second LGTM from any other contributor to this repo before merging.

[raynigon]

LGTM

[gmlewis]

Thank you, @raynigon !
Merging.