pd.to_numeric(series, downcast='integer') does not prpoerly handle floats over 10,000
Hi - I came across this issue in [stackoverflow ](http://stackoverflow.com/a/38983440/6041010)while testing pd.to_numeric()

> If all floats in column are over 10000 it loses precision and converts them to integers.

```
tst_df = pd.DataFrame({'colA':['a','b','c','a','z', 'q'],
                      'colB': pd.date_range(end=datetime.datetime.now() , periods=6),
                      'colC' : ['a1','b2','c3','a4','z5', 'q6'],
                      'colD': [10000.0, 20000, 3000, 40000.36, 50000, 50000.00]})

pd.to_numeric(tst_df['colD'],  downcast='integer')
```
![image](https://cloud.githubusercontent.com/assets/16763201/21388415/cc82243c-c741-11e6-9000-3eea48faa7ec.png)

This doesn't seem like the desired behavior.