subscriptions does not respect Organic Groups privacy

This is for 4.7-dev.

Here's the bug:

Basically, blogs post that are supposed to be private to a specific organic group are nevertheless sent [via subscriptions.module] to anyone who has subscribed to the originating poster's blog... regardless of whether they are a member of the "private organic group".

More specific breakdown of behavior:

1. A user [call them "fanboy"] subscribes to another user's blog [call them "superstar"]
2. User "superstar" posts a blog to a specific audience in an organic group [call this group "The VIPs"] and marks the checkbox to indicate that this posting should *not be public*
3. "fanboy" *still* gets an email from the subscriptions.module informing them that "superstar" has just written a blog... even tho' "fanboy" is not even a member of ["The VIPs"].

4. Riot on the internet ensues :-)

Thanks for your attention to this. I'm not sure if this behavior also occurs in 5.0 [either v1 or 2]... though I suspect it might. Regardless, this is a significant issue. Thanks!