Don't emit a warning with `.rename` if it is a no-op

[hoxbro]

What is your issue?

When running this code:

import xarray as xr
ds = xr.tutorial.load_dataset("rasm")
ds.rename({"time": "time"})

I will get the following warning:

UserWarning: rename 'time' to 'time' does not create an index anymore. Try using swap_dims instead or use set_index after rename to create an indexed coordinate.
  ds.rename({"time": "time"})

[welcome]

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[max-sixty]

I think we'd merge a contribution here. But for the sake of prioritization — this warning on .rename will likely go away at some point...

[hoxbro]

I'm up for making a PR.