gensupport: revert change to send with context
gensupport: revert change to send with context

This reverts a change that removed the usage of the
custom send-with-context. Although req.WithContext is
now usable, it does not return context.Canceled -
it returns an error like "[...] context canceled".

That constitutes a breaking API change, so let's
continue using the old behavior.

Change-Id: Ic89974880c8758bf0e10f1054b02e8e90e5aec19
Reviewed-on: https://code-review.googlesource.com/c/35371
Reviewed-by: Eno Compton <ad9228447a2ef39a5712f4b80b45071fa843fa0d@google.com>

googleapis/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client