Should stream / iter_contents be used when using Download

[dhermes]

The primary motivation is that users could avoid putting the contents into RAM all at once and just flush right to disk (Context). requests has support for a "lazy" response body.

/cc @lukesneeringer @jonparrott @tseaver

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It's worth noting that I have a goal of making google.resumable_media.FOO subpackages that explicitly support a given transport (mentioned in first release notes). Once something like this exists, it makes more sense to add in transport-specific optimizations.