Ignore requests with X-Requested-With:XMLHttpRequest

I've been working on organizing the support queue a bit. One common problem is with ajax requests. We provide some default patterns in the ignore list like */autocomplete/*, but the defaults miss many contrib modules that have unpredictable paths.

It occurs to me that we could test for the header X-Requested-With:XMLHttpRequest and always ignore requests with this header. This should greatly reduce compatibility problems with contrib modules.