Added test + fix for belongs_to to define an assoc setter based on name.

Change-Id: I67fa9ff0288997f6182be7305047469cb44b7ee4
Reviewed-on: http://review.couchbase.org/41659
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>