np.nan rows being dropped from HDFStore frame_tables using append
When storing a dataframe using append, np.nan rows are dropped from the stored frame_tables:

``` python
test = pd.Series(range(401125), dtype=np.float64)
idx = np.random.permutation(test.index)[:50000]
test[idx] = np.nan

# check that there are indeed np.nan in the series
test.isnull().value_counts()
False    351125
True      50000

# create a store
tmp = pd.HDFStore('tmp.h5', 'w')

# add as a DataFrame using put
tmp.put('df_put', pd.DataFrame(test))

tmp.df_put.squeeze().isnull().value_counts()
False    351125
True      50000

# add as a DataFrame using append
tmp.append('df_append', pd.DataFrame(test))

tmp.df_append.squeeze().isnull().value_counts()
False    351125
```
