BUG: already mixed indexes should not sort
cc @jreback

passes for me
 Yeah but not passing on Travis :(
 Some tuple edge case
 @jreback can u take a look here? previously this was sorting (since tuples are sortable), then comparing against a `MultiIndex`. So this _is_ a breaking API change, albeit for non `MultiIndex` `tuple` `Index`es, which are rare.
 the change is that sorting mixed dtype indexes doesn't work for _any_ objects, including things that _do_ have an ordering.
 @jreback ok with this?
 seems ok, I don't really understand what its supposed to do. doubt this is really an issue in practice (except the error of course!) so ok
 `tuple`s have an ordering which means they can be sorted inplace no problem, but i check for `mixed` and `mixed-integer` `inferred_type` which tuples satisfy. i thought it was better to not sort anything that's mixed, than to special case tuples
