Activity: Make sure a non-admin can delete their own activity.

Changes in r8697 broke this functionality due to a strict type check. The
activity user ID is a string, while the logged-in user is an integer.

Commit fixes this by typecasting the activity user ID as an integer and
adds a unit test.

Fixes #5900 (trunk).

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9043 cdf35c40-ae34-48e0-9cc9-0c9da1808c22