Lager doesn't print iolists with ~s right

```
1> list_to_binary(lager_trunc_io:format("iolist: ~s", [[$H, $e, $l, $l, $o, "I", ["am", [<<"an">>], [$i, $o, $l, $i, $s, $t]]]], 1024)).
** exception error: bad argument
     in function  lager_trunc_io:format/2
        called as lager_trunc_io:format("iolist: ~s",
                                        [[72,101,108,108,111,"I",["am",[<<"an">>],"iolist"]]])
2> list_to_binary(oo_lib:format("iolist: ~s", [[$H, $e  $l, $l, $o, "I", ["am", [<<"an>>>], [$i, $o, $l, $i, $s, $t]]]])).
<<"iolist: HelloIamaniolist">>
3> list_to_binary(lager_trunc_io:format("iolist: ~p", [[$H, $e, $l, $l, $o, "I", ["am", [<<"an">>], [$i, $o, $l, $i, $s, $t]]]], 1024)).
<<"iolist: [72,101,108,108,111,\"I\",[\"am\",[<<\"an\">>],\"iolist\"]]">>
```