to_csv case insensitivity
#### Different DataFrames saved under filenames with different cases are overwritten

```python
# Generate three different DFs
lower_df = pd.DataFrame({'a':['spam','eggs','ham']})
camel_df = pd.DataFrame({'a':['Spam','Eggs','Ham']}) 
upper_df = pd.DataFrame({'A':['SPAM','EGGS','HAM']})

# Write out the three DFs to three different CSVs
lower_df.to_csv('ingredients.csv')
camel_df.to_csv('Ingredients.csv')
upper_df.to_csv('INGREDIENTS.csv')
```
#### Problem description

Three different datasets, written to filenames with three different case sensitivities, results in only one file being produced.

#### Expected Output

On Windows 64, only one file is produced: it's titled "ingredients.csv" but contains the data from `upper_df`. I would expect three different files to be written out with the corresponding data from each of the three DFs.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.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: 0.20.3
pytest: 3.2.1
pip: None
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>

  