BUG: fix PeriodConverter issue when given a list of integers (GH9012)

For example coming up when [0, 1] is passed by axhline

Restore old behaviour, but special case arrays of Periods to keep the
performance gain of using PeriodIndex instead of get_datevalue for that case.
TST: period-like test for GH9012
DOC: v0.15.2 editiing, removing several duplicated issues
DOC: update release notes
unstack with nan in the index