Setting the value doesn't work for arrays

If my form state has a value, that is an array: [this] https://github.com/bamlab/react-native-formik/blob/master/src/makeReactNativeField.js#L9 fails

Say the form is like:
```javascript
{
arrayValue: [
{ name: "Hello" }
]
}
```

Indeed you would set `name=arrayValue.${i}.name` on the input and lodash is not getting by path

Ping @Gguigre if you want to add more details