Fixed Issue #2860: dup string if frozen when formatting output for machine.

JSON parser return frozen strings for hash keys. Duplicate frozen strings to avoid the error.

```
vagrant@node01:~/vagrant$ vagrant box list --machine-readable
1390142590,,box-name,precise32
1390142590,,box-provider,virtualbox
1390142590,,box-info,url,http://files.vagrantup.com/precise32.box
1390142590,,box-info,downloaded_at,2014-01-19 14:04:53 UTC
```