Strange reindex behavior
```
regressors = regressors.reindex(index=factors.index)
logger.debug('Regressors have the same index as factors: {}'.format(regressors.index is factors.index))
logger.debug('Regressors and factors indexes are the same: {}'.format(all(map(lambda x, y: x == y, regressors.index, factors.index))))
```

Results in this:

```
2014-06-20 10:47:59,747 DEBUG    pmt.portfolio_analytics.attribution.AlphaData - Regressors have the same index as factors: False
2014-06-20 10:48:00,033 DEBUG    pmt.portfolio_analytics.attribution.AlphaData - Regressors and factors indexes are the same: True
```

Why?
