add XHR response headers to beacon if available
add XHR response headers to beacon if available

We do not set headers at this point, but perhaps we should set headers
on HTTP error.  At this point Headers are too large to beacon.

sdgdsffdsfff/boomerang,lognormal/boomerang,FredXue/boomerang,lognormal/boomerang,levexis/boomerang,sdgdsffdsfff/boomerang,FredXue/boomerang,levexis/boomerang