BUG: DatetimeIndex with freq raises ValueError when passed value is short
When `DatetimeIndex` is created by passing `freq` and `check_integrity=True`, passed `freq` and `inferred_freq` must be identical. But this comparison can fail if length of passed data doesn't have enough length to infer freq. 

Added additional logic to determine whether passed values are on the passed `freq`.

this is fine, rebase and can go in
 thanks!
