BUG: Mixed period cannot be displayed with ValueError
- [x] closes #xxxx (not reported)
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

```
# we can create Series contains Periods with mixed freq
s = pd.Series([pd.Period('2011-01', freq='M'), pd.Period('2011-02-01', freq='D')])

# print the created Series raises ValueError
print(s)
# ValueError: Input has different freq=D from PeriodIndex(freq=M)
```
