Unexpected in-place changes when saving a DataFrame to Stata with write_index=False
After executing `df.to_stata()` with `write_index=False`, all `NaNs` in `df` were automatically replaced by `8.988466e+307`. Please see the following code:

`df = pd.DataFrame(np.random.randn(5,4), columns=list('abcd'))`
`df.ix[2, 'a':'c'] = np.nan`
`print df`
`df.to_stata('test.dta', write_index=False)`
`print df`
I am using `pandas v0.16.0`. Thanks.
