Add coverage for retrieving an activity stream

[jessemcbride]

Right now we wrap get_activity_stream_summary() but not get_activity_stream().

We should include that.

[Thetwam]

This endpoint isn't covered due to a limitation of our current implementation of PaginatedList. PaginatedList expects objects of a single type, but this endpoint returns objects of mixed types. To cover this endpoint, we'll need to improve PaginatedList.