to_csv headers kwarg now works regardless of index flag
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Merge pull request #6916 from marcua/to-csv-headers

to_csv `headers` kwarg now works regardless of `index` kwarg
ENH: Implement Panel.pct_change

Had to change the implementation in generic.pct_change
to not pass `periods` as a keyword arg to generic.
Once the signitures of DataFrame.shift() and Panel.shift()
are aligned, this change can be reverted.