Ruby Client for Faraday: fix file downloading

[lumpidu]

In MR #16876, a bug was introduced that causes file downloading to fail for the Faraday adapter.

@meganemura @dkliban

This commit fixes the obvious missing parameter for the method call to download_file() and the missing variable request for saving the response value of the call to build_request().

Tested so far only locally, but would like to add an appropriate test case for binary data download but need some feedback, how to integrate this kind of tests into this MR.

There is an appropriate test scenario for the Python client that could be used as reference:

https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/echo_api/python/test/test_manual.py#L110

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[wing328]

thanks for the PR.

cc @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

thanKs again for the fix. I'll add a test or two after adding this PR.

[wing328]

filed #17362 to add tests to ensure the file download works as expected in all ruby libraries supported