[Finish #26789753] Mandrill API requires a name, so we're just sending the email address twice to satisfy that requirement
[#26789753] To argument must be wrapped in an Array to work
[#26789753] To argument must be wrapped in an Array to work
[#26789753] Adding tests to ensure payload is correct
[#23694459] Mandrill API response stores the api response in the appropriate settings key, so the Mail gem will pick this up and return it if you call .deliver! instead of just .deliver.  The former will return the API response; the latter the Mail object
Update mandrill delivery handler to accept a from-name in the message header
[Finish #29193013] Adding support for Mandrill API's Reply To email header feature
re-implementation of @highgroovedan's API update patch

* The patch in [this](https://github.com/mailchimp/mailchimp-gem/pull/7) pull
  request provides important functionality but the implementation exposed
  internal implementation for the sake of testing.  Instead I've nested it away
  as a private method and just called it directly using `#send`.  While testing
  private methods is something of a no-no, I prefer this to exposing an
  interface where one should noe be.