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 
it DID solve it. Beacuse the subsequent routines basically stringify numbers (e.g. column headings by putting a space before it). But not entirely sure why/how that is.