Column is not converting to numeric when errors=coerce
I read in my dataframe with

```
 pd.read_csv('df.csv')
```

And then I run the code:

```
df['a'] = pd.to_numeric(df['a'], errors='coerce')
```

but the column does not get converted. When I use errors = 'raise' it gives me the numbers that are not convertible but it should be dropping them with coerce.... This was working perfectly in Pandas 0.19 and i Updated to 0.20.3. Did the way to_numeric works change between the two versions?

