Allow mails to be sent to multiple recipients

I did this pull request because mandrill_integration does not allow to send email to multiple recipients, the json generated here https://github.com/AlessioRocco/mandrill_integration/compare/fix_multiple_email?expand=1#diff-0afe58b2581acf29a39eef1e2c5a28eaL38 was like this:

```
"to": { email: "email1@example.com, email2@example.com "}
```

instead in the [mandrill documentation](https://mandrillapp.com/api/docs/messages.JSON.html#method=send-template) the format should be:

```
"to": [{ email: "email1@example.com"}, { email: "email2@example.com "}]
```
could we have a to_s here just to make sure it doesn't blow with undefined nil class method?