Port r7254 from 1.7 branch to trunk - setcookie tweaks

"Only delete the bp-message and bp-message-type cookies when they are currently set. Otherwise, Set-Cookie headers are always issued, which messes with caching proxies."

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7260 cdf35c40-ae34-48e0-9cc9-0c9da1808c22