Output filter invoked twice when using mod_rewrite and mod_xsendfile

[sosherof]

When using mod_rewrite to run a PHP script that inserts an X-Sendfile header to be use by the mod_xsendfile module, mod_rewrite is invoked a second time for a subrequest during file path validation. That's fine but the second invocation causes the hook (handle_bw) to add the bandwidth output filter twice. I haven't verified if that actually causes duplicate data on the wire but it causes BW to think there's twice as many connections, causing it to apply limits unnecessarily.

I've got a branch I'll submit soon with a fix, along with the PR. Hopefully that can be incorporated into the new version.