Bug in DataFrame.to_html with MultiIndex

[tkf]

DataFrame(
    rand(2, 4),
    columns=MultiIndex.from_tuples(zip('aabb', 'xyxy'))
    ).to_html()

generates

a x	a y	b x	b y
0	0.280664	0.465508	0.616751	0.246239
1	0.124957	0.867052	0.504789	0.178313

[lodagro]

see also #909, issue only pops up when columns use MultiIndex without level names.

[tkf]

Thanks!