Exchange 'all' and '' seem inconsistent

This seem to return the prices correctly from SQL that contains trades on multiple exchanges

```
bq.trades_to_price('','btcusd','30min',label='right')
```

but exchange='all' leaves out the last time step:

```
bq.trades_to_price('all','btcusd','30min',label='right')
```

Appears to be a non-obvious bug.
