formatting 'B' from large 'MB' is wrong

```Python
'{0:a}'.format(DataSize('14796MB'))
>>> '13.779849GiB'
'{0:B}'.format(DataSize('14796MB'))
>>> '14796000000EB'
```
should look more like
```Python
'{0:B}'.format(DataSize('14796MB'))
>>> '14796000000B'
```