Mime Mail returns array instead of bool

In #808518: API change: Split mail preparation from sending, Mime Mail module is preparing an API change. Now mimemail() returns a boolean but after this API change will return an array with the whole message. The actual boolean will be included in return array as key "result".

From PHPDoc:

* @return
*   An array containing the MIME encoded message, including headers and body.

Send must adapt to the new Mime Mail API.