Prompting doesn't properly set config values

I did:

```
salty-wordpress ➜  ftp-test.dev  wp core install --prompt
1/5 --url=<url>: ftp-test.dev
2/5 --title=<site-title>: FTP Test
3/5 --admin_user=<username>: daniel
4/5 --admin_password=<password>: daniel
5/5 --admin_email=<email>: d@danielbachhuber
Success: WordPress installed successfully.
```

I expected the site URL to be set to `ftp-test.dev`. It was instead set to `example.com`