Hopefully I'm not bike-shedding here, but is this the best name for this method? `fq_response_header` seems accurate enough, but it's a little abstract and takes me some thought.

I think what hangs me up is that these are the filter queries of the original request. We see them here as an echo in the response, but other than that they're not really a property of the response at all.

Probably not worth bothering about until I get around to redesigning some of these internals for Sunspot 3.