mbcollection error: HTTP Error 400: Bad Request

### Problem

Hey, I was trying to add my music collection to Musicbrainz, but got the following error:

error: MusicBrainz API error: caused by: HTTP Error 400: Bad Request

Looks similar to this earlier bug report: https://github.com/beetbox/beets/issues/1152

Running this command in verbose (`-vv`) mode:

```sh
beet -vv mbupdate
user configuration: /home/stalenbas/.config/beets/config.yaml
data directory: /home/stalenbas/.config/beets
plugin paths:
Sending event: pluginload
library database: /home/stalenbas/.config/beets/musiclibrary.db
library directory: /run/media/stalenbas/BASCH3/Muziek
Sending event: library_opened
mbcollection: Updating MusicBrainz collection a29b7044-9cd2-3749-8b72-fe84b708bc4d...
error: MusicBrainz API error: caused by: HTTP Error 400: Bad Request
```

### Setup

* OS: Arch Linux 64 bit
* Python version: 3.7.2-3
* beets version: 1.4.7-3

My configuration (output of `beet config`) is:

```yaml
directory: /run/media/stalenbas/BASCH3/Muziek/
library: ~/.config/beets/musiclibrary.db
import:
copy: no
write: no

plugins: mbcollection
musicbrainz:
user: [my username]
pass: [my password]

```

Start with the mbcollection plugin's mbupdate entry point and reproduce the reported `beet -vv mbupdate` command using the configuration shown. Inspect the MusicBrainz request that produces HTTP 400 and compare it with the API requirements and the earlier issue. Done means the reported collection update no longer fails with this response and the behavior is verified against the reported setup.

Written by the indexing model from the issue text.