BUG: python and c engines for read_csv treat blank spaces differently, when using converters
#### Code Sample, a copy-pastable example if possible

```
In [109]:
data = np.array([[ 'c1', 'c2'],
                [ '', 0.285],
                [ 10.1, 0.285]], dtype=object)

In [110]:
pd.DataFrame(data).to_csv('test.csv',header=False,index=False)

In [111]:
!cat test.csv
Out[111]:
c1,c2
,0.285
10.1,0.285

In [113]:
pd.read_csv('test.csv',converters={'c1':str},engine='c').values
Out[113]:
array([['', 0.285],
       ['10.1', 0.285]], dtype=object)

In [114]:
pd.read_csv('test.csv',converters={'c1':str},engine='python').values
Out[114]:
array([[nan, 0.285],
       ['10.1', 0.285]], dtype=object)

```
#### Expected Output

Notice that the output for the python engine and the c engine are different.  I am not sure which one is preferable/expected.
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 13.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.1
setuptools: 21.0.0
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.1.2
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.2
pytz: 2016.3
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.6
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.5.0
bs4: 4.4.1
html5lib: 0.9999999
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: 2.39.0
```
### Details

The culpruit seems to be this function: https://github.com/pydata/pandas/blob/master/pandas/io/parsers.py#L1299

Before that call, `results` and `values` both contain the `''` string in the python engine version of the parser.  That call changes the `''`' value in `values` to `nan` and thus changes the value of `results` as well.  Specifically, the change for the python engine happens here: https://github.com/pydata/pandas/blob/0c6226cbbc319ec22cf4c957bdcc055eaa7aea99/pandas/src/inference.pyx#L1009

```
if (convert_empty and val == '') or (val in na_values)
```

It seems that `na_values` contains `''` by default for certain parsing calls.  This means that when `val == ''` triggers the `nan` conversion whether or not `convert_empty` is true.

I haven't tracked down the change in the `c` engine.
