Support creating record with multiple content values

Implement creating a DNS record name with multiple values, this is, creating one entry with the same name and each one with one different value.

Now the LWRP supports both String and Array values when setting content:

```ruby
dnsimple_record 'multiple' do
  type 'A'
  content ['2.2.2.2', '3.3.3.3']
  domain 'example.com'
  username 'user@email.com'
  password 'my123password'
end
```

And creates the following DNS records:

```
multiple A 2.2.2.2 3600
multiple A 3.3.3.3 3660
```