Regression: Dataset.update(Dataset)

[crusaderky]

Dataset().update(Dataset())

FutureWarning: iteration over an xarray.Dataset will change in xarray v0.11 to only include data variables, not coordinates. Iterate over the Dataset.variables property instead to preserve existing behavior in a forwards compatible manner.

This is a regression in xarray 0.10.4.
@shoyer this isn't serious enough to warrant an immediate release on its own, but we're already doing one so we might as well include it.