check_meta(): use __class__ when checking dataframe types

[madsbk]

Similar to #6981, this PR makes check_meta() use x.__class__ instead of type(x) when checking dataframe types.

• Tests added / passed
• Passes black dask / flake8 dask

[mrocklin]

Thanks @madsbk . I'm curious, does it make sense to include a test for this? Otherwise I would expect a change like this to have a decently high probability of being reverted at some point in the future.

[madsbk]

Thanks @madsbk . I'm curious, does it make sense to include a test for this? Otherwise I would expect a change like this to have a decently high probability of being reverted at some point in the future.

Good point, I have been guilty of doing such a revert in the past :)
Will add a test

[madsbk]

Added tests that will fail if x.__class__ isn't used.