Include the `site_url()` in the semaphore key.

Using only the `__METHOD__` would block all requests across a multisite network or shared server, but adding the `site_url()` localizes the block to a single site.


git-svn-id: https://plugins.svn.wordpress.org/tagregator/trunk@1020630 b8457f37-d9ea-0310-8a92-e5e31aec5664