Add Zstd support (optional dependency, just like Brotli)

[adrianodennanni]

Just like #650 (adds brotli compression), this PR aims to give Mechanize support for Zstandard (zstd), in a similar way (will only work if you have the zstd-ruby gem installed).

[flavorjones]

@adrianodennanni Thanks for the pull request. Can you help me understand the specific problem that you're having that led you to create this pull request? Is there a server somewhere that is returning content unexpectedly encrypted with zstd, or that doesn't support any other forms of compression?

[adrianodennanni]

Hello @flavorjones ! We had a case where a request was not accepted because the headers were not exactly equal to a request made from a real browser. So, we informed the Accept-Encoding header with gzip, deflate, br, zstd as value.
Although usually the server chooses the first encoding algorithm to use, this site choose zstd instead, and returned a body that Mechanize could not understand.

[adrianodennanni]

Also, some benchmarks point out that Zstd decompresses 40% faster than Brotli. Informing Accept-Encoding as zstd could also provide performance gains for the client.

[flavorjones]

@adrianodennanni Thanks for the quick response. This looks great! Going to merge it and cut a release shortly.