`config extra` command does not work with --json when a key is a quoted numeric

When I run this command:

```
composer config extra.patches.drupal/typogrify --merge --json '{"3398815":"patches/typogrify.3398815.patch"}'
```

I get this added to composer.json

```
    "extra": {
        "patches": {
            "drupal/typogrify": ["patches/typogrify.3398815.patch"]
        },
```

The "3398815" has been ignored.
