Doesn't properly sort case insensitive strings

By adding after:

```
            aVal = valueOf(a, keyPaths[i].path);
            bVal = valueOf(b, keyPaths[i].path);
```

this:

```
            if (typeof valueOf(a, keyPaths[i].path) == 'string' && typeof valueOf(b, keyPaths[i].path) == 'string'){
                aVal = aVal.toLowerCase();
                bVal = bVal.toLowerCase();
            }
```

fixes the issue :)
