varnish: allow POST for EventLogging on bits

The sendBeacon W3C API works by issuing POSTs to EventLogging URLs.
Currently, we issue a "method not allowed" on POSTs; while that would
still work with EventLogging (as neither the browser nor EL would care
about the status code), this would elevate our error statistics. Handle
this case by moving the conditional block and thus allowing POSTs
specifically for event.gif.

RT: 8612
Change-Id: I7d752d1fe84620ed64101d6bece7631057a8a3e6