BUG: Bug in to_datetime when format='%Y%m%d and coerce=True are specified, where previously an object array was returned (rather than
     a coerced time-series with NaT), (GH7930)
Merge pull request #7931 from jreback/dt_format

BUG: Bug in to_datetime when format='%Y%m%d and coerce=True are specified (GH7930)
BUG: rolling_* treatment of center