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 

Not sure if this is the same issue, but I'm getting weird behavior on MultiIndex displays (and possibly in the underlying sortin too) as described in #8017 in my 10/29 update.
