Load right stream with Show More on mobile

Fix issue #4999

  - Load aspects stream if user is on this stream instead of loading the
    activity stream
Don't mix old and new style hash syntax. also no need for the parens around the or:

```ruby
aspect_ids = session[:a_ids] || []
aspects_stream_path(max_time: time_for_scroll(@stream), a_ids: aspect_ids)
```

I'd also try if it works if set to nil instead of empty array:
```ruby
aspects_stream_path(max_time: time_for_scroll(@stream), a_ids: session[:a_ids)
```

Finally specs! ;) https://github.com/diaspora/diaspora/blob/develop/spec/helpers/stream_helper_spec.rb#L27