Handle encoding value of "none"

[GusPrice]

When a file is over 1 MB in a repository, GitHub responds with "none" as the encoding value. Handle this accordingly.

[WillAbides]

If I understand the docs correctly, Content should always be nil when encoding is "none". I think it would be better to continue returning an error on "none", but it could be a better error that explains that this is because the file size is over 1MB and suggests using DownloadContents instead.

[GusPrice]

Ah! It seems in fact I have misunderstood! Definitely would be nice to have a better error though. I'll update my PR accordingly.

[GusPrice]

Updated to give a nicer error message in that case!

[WillAbides]

LGTM. Just a small nit about the test case, but you don't necessarily need to take my suggestion there.

To set expectations, the primary maintainer is out until 9/18, so this PR is not likely to be merged before then.

[codecov]

Codecov Report

Merging #2924 (bee9a1a) into master (fd33b81) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head bee9a1a differs from pull request most recent head 6e8946c. Consider uploading reports for the commit 6e8946c to get more accurate results

@@ Coverage Diff @@## master #2924 +/- ##
=======================================
Coverage 98.17% 98.17% =======================================
Files 143 143 Lines 12609 12611 +2 =======================================
+ Hits 12379 12381 +2 
Misses 156 156 Partials 74 74 

Files Changed	Coverage Δ
github/repos_contents.go	87.97% <100.00%> (+0.15%)	⬆️

[gmlewis]

Thank you, @GusPrice and @WillAbides !
LGTM.
Merging.