Fix bug with Model.find_by_id([id])

Model.find_by_id, when called with an array containing a single argument
would return a single instance of the model, rather than the exepected
single item array.

Change-Id: Iffd7a5c93ecc02b952f6b6badfdbf9ee5b3df021
Reviewed-on: http://review.couchbase.org/26591
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>