Additional condition for direct comparison creating `DatetimeIndex` (`on_freq`) in case of inferred freq mismatch.

Currently, following `DatetimeIndex` creation fails, for example.
```
import pandas as pd
idx1 = pd.date_range('2011-01-1', periods=2, freq='D')
# [2011-01-01, 2011-01-02]
# Length: 2, Freq: D, Timezone: None

idx2 = pd.DatetimeIndex(idx1, freq='D')
# ValueError: Dates do not conform to passed frequency
```