[Table] Header row has the wrong min-height

Bug, feature request, or proposal:

bug

What is the expected behavior?

According to the guidelines, the height of the header row of a data table should be 56px.

https://material.io/guidelines/components/data-tables.html#data-tables-specs

What is the current behavior?

The header row has the same height as a normal row (min-height: 48px).

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