64bit read_csv with dtype problem
#### Code Sample, a copy-pastable example if possible

```
test_str = u'1.7976931348623157e+308,1.7976931348623157e+308'
ts_dbl = u'1.7976931348623157e+308'
print np.float64(ts_dbl)
dt={}
tmpio = StringIO(test_str)
market_head_list = [ "ClosePrice", "SettlementPrice"]
dt['ClosePrice']=np.float64
dt['SettlementPrice']=np.float64
df =pd.read_csv(tmpio,engine = 'c',names=market_head_list,dtype=dt)
```
#### Expected Output

```
data = self._reader.read(nrows)
```

  File "pandas\parser.pyx", line 805, in pandas.parser.TextReader.read (pandas\parser.c:8620)
  File "pandas\parser.pyx", line 827, in pandas.parser.TextReader._read_low_memory (pandas\parser.c:8876)
  File "pandas\parser.pyx", line 904, in pandas.parser.TextReader._read_rows (pandas\parser.c:9893)
  File "pandas\parser.pyx", line 1011, in pandas.parser.TextReader._convert_column_data (pandas\parser.c:11286)
  File "pandas\parser.pyx", line 1092, in pandas.parser.TextReader._convert_tokens (pandas\parser.c:12659)
ValueError: cannot safely convert passed user dtype of <f8 for object dtyped data in column 0
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 18.5
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.16.0
statsmodels: None
xarray: None
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.5.3
pytz: 2015.7
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.0
openpyxl: 2.2.6
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.7
lxml: 3.4.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.38.0
pandas_datareader: None
