Issue with specify 'X' template and no template at all to arrow.get

One of arrow's features say that it's UTC by default, but I was running into an issue when parsing unix timestamps:

```python
In [25]: arrow.get('1406430900', 'X').timestamp
Out[25]: 1406416500

In [26]: arrow.get('1406430900').timestamp
Out[26]: 1406430900
```

When specifying the 'X' template arrow was using local time zones, but without the template it was defaulting to the correct behavior (using UTC).