Space before wp_stream_screen class

[Luke Carbis]

I first noticed this when I saw that the date filter was not wide enough to display it's options without wrapping:

Then I noticed some other strange styles in Stream Reports:

Then I realised that this was only happening when Jetpack was enabled. Looking into this further, I realised that when Jetpack adds it's body class (i.e. jetpack-disconnected) it does so without adding a space at the end.

This resulted in a body class that looked like this: jetpack-disconnectedwp_stream_screen

We should add a space before our body class to prevent this from happening.