Fix share key pattern matching

Fixes https://github.com/owncloud/core/issues/11127

But there's another failing pattern, unit test will fail.
Still looking for a solution...
Think about two users, "name2" and "name1.name2". You want to delete all shares keys for "name2". With the preg_match statement above you would also delete the share key for user "name1.name2", right?