BUG: make sure that the multi-index is lex-sorted before passing to _lexsort_indexer (GH8017)

BUG: sparse repr of multi-index frame with a FloatIndex as a level was incorrect

closes #8017 
ah, it did solve something else as the wrong repetition of the first level? like https://github.com/pydata/pandas/issues/8017#issuecomment-55868651