Automatically Object.assign when `styles` receives an Array

In React Native, these two styles are automatically composed:

```
<Text style={[styles.ratingValue, getStyleFromScore(criticsScore)]}>
```

Currently, trying a similar technique in React DOM produces this:

```
<span style="0:[object Object];1:[object Object];">
```

Following the principles of "Learn Once, Write Everywhere", the style prop should behave similarly across implementations.
