DataFrame.to_csv(quoting=csv.QUOTE_NONNUMERIC) quotes numeric values for dot separators
#### Code Sample, a copy-pastable example if possible

```python
def test_pandas():
    import csv
    import pandas as pd
    import numpy as np

    df = pd.DataFrame.from_dict({'column': [1.0, 2.0]})
    assert df['column'].dtype == np.dtype('float')
    outpath = 'out.csv'

    df.to_csv(outpath, quoting=csv.QUOTE_NONNUMERIC, sep=".", index=False)  # This fails
    # df.to_csv(outpath, quoting=csv.QUOTE_NONNUMERIC, sep="!", index=False)  # This is fine
    # df.to_csv(outpath, quoting=csv.QUOTE_NONNUMERIC, sep=":", index=False)  # This is fine
    # df.to_csv(outpath, quoting=csv.QUOTE_NONNUMERIC, sep=",", index=False)  # This is fine

    with open(outpath,"r") as f:
        f.seek(0)
        lines = f.read().splitlines()
        assert lines[0] == '"column"'
        assert not lines[1].startswith('"')  # <--- 'sep="."' crashes here
        assert [1, 2] == list(map(float, lines[1:]))

test_pandas()
```
#### Problem description

Using a "." as a separator causes QUOTE_NONNUMERIC to quote floats, even if there is only one column.

Explicitly stating other separators (I tried a couple) is fine.

#### Expected Output

The test above should pass.

#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.3
numpy            : 1.16.5
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 41.6.0.post20191030
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.11
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
```

</details>
