Failing Unit Test and Patch for Fixing MonthEnd DateRange Unions with Timezones
This patch preserves the timezone information when applying an offset to `DatetimeIndex` with a timezone applied.  The `MonthEnd` Offset wasn't retaining the timezone information resulting in an error even when both indexes had timezones.

```
TypeError: can't compare offset-naive and offset-aware datetimes
```

merged as ce8a15d5ea1f6c.

Thanks.
