TST/BUG: HDFStore.append_to_multiple with dropna=False buggy
xfaile this test: https://github.com/pandas-dev/pandas/pull/15601/commits/4dc2ef8b0af84e60e8ec1dcc070dc9e478defdf8

in ``pandas/tests/io/test_pytables.py``
```
    @pytest.mark.xfail(run=False,
                       reason="append_to_multiple_dropna_false "
                       "is not raising as failed")
    def test_append_to_multiple_dropna_false(self):
        df1 = tm.makeTimeDataFrame()
        df2 = tm.makeTimeDataFrame().rename(columns=lambda x: "%s_2" % x)
        df1.iloc[1, df1.columns.get_indexer(['A', 'B'])] = np.nan
        df = concat([df1, df2], axis=1)

        with ensure_clean_store(self.path) as store:

            # dropna=False shouldn't synchronize row indexes
            store.append_to_multiple(
                {'df1a': ['A', 'B'], 'df2a': None}, df, selector='df1a',
                dropna=False)

            with pytest.raises(ValueError):
                store.select_as_multiple(['df1a', 'df2a'])

            assert not store.select('df1a').index.equals(
                store.select('df2a').index)
```

This sometimes succeed and sometimes fails. Needs some investigation.