Column multi-index not restored correctly on load from HDF5 table
v0.12.0

cols = pd.MultiIndex.from_tuples([(1,2), (3,4)])
data = pd.DataFrame(index=[1,2,3], columns=cols, dtype=float)
data.values.fill(0.)

s = store('/tmp/test', mode='a')
s.append('mi_frame', data)

reloaded = s['mi_frame']     # also applies to s.select...
print type(data.columns)
Out: pandas.core.index.MultiIndex

print type(reloaded.columns)
Out: pandas.core.index.Index

 It's easy enough to re-convert upon reload, but would be nice not to have to
