BUG: .astype of Float64Index

[jreback]

xref #12866

data = Index([u'0', u'1', u'2'], dtype='object')
data.astype('float64').astype('int64')
TypeError: Setting <class 'pandas.indexes.numeric.Float64Index'> dtype to anything other than float64 or object is not supported

this should work

[jreback]

cc @Komnomnomnom