Fix uia type string on iOS 8 webviews

```
# Typing in a UIWebView iOS 8
{"status"=>"success", "index"=>2}

# Typing in a UIWebView iOS 7
{"status"=>"success", "value"=>":nil", "index"=>0}
```

The result is that uia_type_string is raising an error because it expects 'value'.

### Todo

- [ ] @krukow Needs review.  I am not sure I understand why no 'value' is returned.  Is this expected?  Something we should fix upstream?