[Ruby] add file download tests

[wing328]

a follow-up PR to #17333

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[wing328]

travis CI failure not related to this change

[lumpidu]

I see that you have also fixed an error in the Typhoeus adapter. I used this adapter before and needed to switch to Faraday because after my upgrade to the latest OpenApi generator, this adapter stopped working.

Thx for fixing !

[wing328]

@lumpidu thanks Daniel. Sorry for the regression. We should have added the test right away but missed it as there are too many pending works.

Thanks again for the PR.