MultiIndex promotes float64 dtype to object
``` python
import pandas as pd
import numpy as np
print pd.__version__
#0.11.0.dev-184f2db

lev1 = np.arange(10)
lev2 = np.linspace(0, 9, 10)
mi = pd.MultiIndex.from_arrays([lev1, lev2])

# ints work fine
assert lev1.dtype == mi.levels[0].dtype

# failure
assert lev2.dtype == mi.levels[1].dtype
print lev2.dtype, mi.levels[1].dtype
# float64 object
```

Ran into this when using `np.isclose` on the level values. Numpy fails due to casting rules and object dtype. 
