Client::sendHeadRequest: allow 204 status code

[aspaseltiner-vimeo]

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From the Tus 1.0.0 specification:

The Server SHOULD acknowledge successful HEAD requests with a 200 OK or 204 No Content status.

The current implementation is being overly restrictive. In theory this is also only a SHOULD (so maybe a wider array of statuses should also be allowed), but this is at least an improvement.

Fixes #455 .