Don't break on top-level non-numeric object keys

Previously, this threw (in `__DEV__` only) in `ReactCurrentOwner.current.constructor.displayName` because `ReactCurrentOwner.current` is null:

```
React.renderComponent(<div>{{1: <div />, 2: <div />}}</div>, document.body);
```
Merge pull request #1334 from spicyj/non-numeric-top-level

Don't break on top-level non-numeric object keys