document_setter does not update corresponding id field in belongs_to_document relationship

[ozydingo]

t2 is an object that belongs_to_document :media_file_datum

2.2.2 > t2.media_file_datum_id
 => <BSON::ObjectId:0x70280566925080 data=562c40fc616e64235b000001>
2.2.2 > t2.media_file_datum = MediaFileDatum.new(content: tt.media_file_datum.content)
2.2.2 > t2.media_file_datum_id
 => <BSON::ObjectId:0x70280379751960 data=562c40fc616e64235b000001>
2.2.2 > t2.media_file_datum.id
 => <BSON::ObjectId:0x70280566955560 data=5640caae416e64d29a00000c>

Expected behavior is that calling t2.media_file_datum= should set the media_file_datum_id field as well, which occurs for Rails (4) belongs_to associations. I suspect a similar situation for has_one as but I don't have that setup currently. I'm happy to work on a PR for this perhaps by next week.