to_csv method doesn't work well when use space separator   
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd
s = pd.Series(np.random.randn(5), index=['a', 'b', 'c', 'd', 'e'])
s.to_csv('test.txt',sep=' ',float_format='%12.5f')
```
#### Problem description
The contents of test.txt is 
```
a "        0.09048"
b "        0.13346"
c "        0.13251"
d "        0.00243"
e "        0.03104"
```
not sure where is the " from
#### Expected Output
same result without "
#### Output of ``pd.show_versions()``
<details>
python: 3.5.0.final.0
OS:Linux
pandas: 0.20.3
</details>
