Shard maps for reserved database names cannot be updated

I'm still seeing the same problem as described in COUCHDB-2424 with CouchDB 2.0.0 (Ubuntu) but it doesn't appear that I can reopen that issue.

What I'm trying to do trying to get a newly added cluster node to participate in all the databases already created in the cluster so that I can then remove an existing node and replace it with the new one. The workflow I have is:

• Add the new node to the cluster using the _nodes endpoint. This works.
• For each database edit the shard metadata document (e.g. _db/db_name) and add all shards to the new node.

This appears to work for all databases except the system databases. Essentially this means that you can't move the shards for these database off a node in order to retire that node.

An easy way to reproduce the issue is to open http://localhost:5986/_utils/#/database/_dbs/_metadata and then click "Save Changes" in Fauxton. The error "Save failed: Only reserved document ids may start with underscore." is displayed. Doing the same process for a user database works fine. Similarly, editing the document via curl also fails with the same error.