DataFrame to_csv() ignores nanRep argument for float32 input
import numpy as np
import pandas

d = np.random.randn(8, 4)
df = pandas.DataFrame(d)
df[1][:] = np.nan
df.to_csv('pandas-success.csv', nanRep='999')

d = np.asarray(np.random.randn(8, 4), dtype=np.float32)
df = pandas.DataFrame(d)
df[1][:] = np.nan
df.to_csv('pandas-fail.csv', nanRep='999')

After running this with pandas 0.4.1 the file pandas-success.csv
contains the nanRep string '999' as defined, but the file
pandas-fail.csv contains the string 'nan' instead.
