fix: relax poison and mime gax dependencies

[tomtaylor]

This allows compatibility with other projects that require later versions of the poison and mime libraries.

Also fixed a couple of tests that were not accounting for the "accept-encoding" header that Tesla sends.

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[brunohkbx]

@dazuma @chingor13 can we get eyes on this one please? relevant issue: https://github.com/googleapis/elixir-google-api/issues/8750