Failed to add/update records when using MongoDB backend

When I submit tasks to either direct or load balanced view using either map or apply_sync I see debug logs below:

```
2018-09-18 23:27:05.936 [IPClusterStart] 2018-09-18 23:27:05.935 [IPControllerApp] ERROR | DB Error updating record u'e5925b97-b0906f91a3415ce89c3c2299'
2018-09-18 23:27:05.936 [IPClusterStart] Traceback (most recent call last):
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/ipyparallel/controller/hub.py", line 693, in save_queue_result
2018-09-18 23:27:05.937 [IPClusterStart] self.db.update_record(msg_id, result)
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/ipyparallel/controller/mongodb.py", line 87, in update_record
2018-09-18 23:27:05.937 [IPClusterStart] rec = self._binary_buffers(rec)
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/ipyparallel/controller/mongodb.py", line 68, in _binary_buffers
2018-09-18 23:27:05.937 [IPClusterStart] rec[key] = list(map(Binary, rec[key]))
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/bson/binary.py", line 147, in __new__
2018-09-18 23:27:05.937 [IPClusterStart] raise TypeError("data must be an instance of bytes")
2018-09-18 23:27:05.937 [IPClusterStart] TypeError: data must be an instance of bytes
```

Looks like rec['buffers'] cannot be binarized?

I am running on ubuntu 17.04 with anaconda python 2.7.15, ipyparallel 6.2.2 and pymongo 3.7.0.

從 ipyparallel/controller/mongodb.py 開始，特別查看 _binary_buffers 和 update_record，然後追蹤 ipyparallel/controller/hub.py 中的 save_queue_result 如何提供 rec['buffers']。使用 MongoDB 後端重現一個 map 或 apply_sync 工作，並確認新增或更新記錄能在不出現 BSON bytes TypeError 的情況下完成。

由索引模型根據 Issue 內容生成。