Pandas to_json changing dtype
I noticed this behavior, not sure it's a bug.
I create a dataframe with 2 integer columns and 1 float column

``` python
import pandas as pd
df = pd.DataFrame([[1,2,0.2],[3,2,0.1]])
df.info()
```

```
<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 3 columns):
0    2 non-null int64
1    2 non-null int64
2    2 non-null float64
dtypes: float64(1), int64(2)
```

If I output that to Json, the dtype information is lost:

``` python
df.to_json(orient= 'records')
```

```
'[{"0":1.0,"1":2.0,"2":0.2},{"0":3.0,"1":2.0,"2":0.1}]'
```

All data is converted to float. This is a problem if for example one column contains ns timestamps, because they are converted to exponential notation and the sub-second information is lost.

Can you help?
