to_json converts to UTC when encoding ISO formatted datetimes
#### Code Sample

```

In [99]: ts = Timestamp('2013-01-10 00:00:00-0500', tz='US/Eastern')

In [100]: pd.io.json.dumps(ts, iso_dates=True)
Out[100]: '"2013-01-10T05:00:00.000Z"'

```
#### Expected Output

```
In [100]: pd.io.json.dumps(ts, iso_dates=True)
Out[100]: '"2013-01-10T00:00:00.000-05:00"'

```

cf 

```
In [109]: ts.isoformat()
Out[109]: '2013-01-10T00:00:00-05:00'
```
