fixing array.length only when array of objects

cc @ianstormtaylor for review
I think it might be better to implement it as:

```js
if (is.array(val) && some(val, is.object)) props[key] = val.length;
```

Using `yields/some`, so that we catch any invalid arrays instead of just checking the first item?

But even then, we probably want to do it only if there aren't just simple types in the array? Like an array of arrays should also be converted? Probably just strings and numbers should remain. Don't even know if Mixpanel supports numbers?