Keys not quoted in filter config

This YAML
```yaml
 - plugin_name: mutate
   copy: { host.name: hostname }
```
should generate this config
```ruby
mutate {
    copy => {
        "host.name" => "hostname"
    }
}
```
but instead it generates
```ruby
mutate {
    copy => {
        host.name => "hostname"
    }
}
```
which causes
>FailedAction: Expected one of #, =>