Fix FORMAT ERROR when format a unicode string

lager doesn't support unicode format, either in FmtStr or in Args. this is mainly because lager_stdlib:string_p/1 can't handle unicode strings. (Rune > 255)

this fix try to make "~ts" format works **when compiled with -D UNICODE_AS_BINARIES**

```
%% current version: 
lager:log(info, self(), [20013,25991,27979,35797]). % error
lager:log(info, self(), "~ts", [[20013,25991,27979,35797]]). % error
%% this fix
lager:log(info, self(), [20013,25991,27979,35797]). % error, not support
lager:log(info, self(), "~ts", [[20013,25991,27979,35797]]). % works
```

[20013,25991,27979,35797] is 中文测试, which means "Chinese Character Test".