Preserve column ordering in Dataset.rename_column
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Not sure why CI isn't triggered.

@lhoestq Can you please help me with this? 
2021-03-15T14:38:52Z
https://github.com/huggingface/datasets/pull/2045#issuecomment-799472580
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2021-03-15T14:38:52Z
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I don't know how to trigger it manually, but an empty commit should do the job
2021-03-15T19:04:47Z
https://github.com/huggingface/datasets/pull/2045#issuecomment-799677118
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Currently `Dataset.rename_column` doesn't necessarily preserve the order of the columns:
```python
>>> from datasets import Dataset
>>> d = Dataset.from_dict({'sentences': ["s1", "s2"], 'label': [0, 1]})
>>> d
Dataset({
    features: ['sentences', 'label'],
    num_rows: 2
})
>>> d.rename_column('sentences', 'text')
Dataset({
    features: ['label', 'text'],
    num_rows: 2
})
```
This PR fixes this.