Data sources on new client

[danielballan]

Bug, found by @genematx:

In [9]: c = from_profile('local', api_key='secret')

In [10]: arr = c.write_array([1,2,3], key='arr')

In [11]: arr.data_sources()
...
TypeError: 'ArrayStructure' object is not subscriptable

We had unit-tested c['arr'].data_sources() but did not have a test covering calling data_sources() directly on the object return by write_array(...), in the case where include_data_sources=True had not been set on the client and thus the data sources needed to be fetched.

This PR adds a unit test for that case, which reproduces the bug and fails. It then adds a fix.

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section