Don't be greedy about adding the 'buddypress' class to pages

Fixes an issue where the 'buddypress' body class was being added to nearly
every page, even if it wasn't a BP page by the normal definition.

Fixes #4928

Props dcavins

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6921 cdf35c40-ae34-48e0-9cc9-0c9da1808c22