Allow hashes, arrays or other values as POST body

Allow hashes, arrays or other values as raw POST body.
    
* If the parsed body is a Hash we just return the hash.
* If the parsed body is an Array we return { 'body.multi' => array }
* If the parsed body is other value we retur { 'body.value' => value }
We can have the ruby object there and `Rollbar::Notifier#dump_payload` will convert it to `String`. https://github.com/rollbar/rollbar-gem/blob/master/lib/rollbar.rb#L594