Changing a Group's name through the action api disassociates it from its datasets in the index

[icmurray]

Repro:

• Create a new Group, named "test-group".
• Add a dataset to it.
• Verify the dataset belongs to the group by visiting the Group's read-page
• Update the Group through the action api (group_update), using the uid in the "id" field, and a new name in the "name" field.
• Visit the group's read-page. The list of datasets will be empty.

This was an issue when editing a Group through the web interface, which was fixed in [1]. However it only fixes the issue in the group controller.

[1] ​https://github.com/okfn/ckan/commit/dbe25d8b8d7fabfc40c5d794a920b91cec349335