BUG: Mismatch between get and set behavior for slices of floating indices (fixes #2727)
fixes #2727: `_NDFrameIndexer._convert_to_indexer` modified to match behavior in `_NDFrameIndexer._get_slice_axis`, new tests added to existing unit test

Was informed that I could have forced the rebased push through rather than making a new branch, will do that next time
 cherry-picked. thanks
